In the journey toward the pinnacle of audio reproduction, we encounter a fundamental paradox. It is the yearning for an amplifier that possesses infinite driving power while simultaneously embodying absolute characterlessness—complete silence. Might not the ideal amplifier be one that proves its worth not through what it adds, but rather through its very “absence”? Rather than adding any interpretation to the sanctity of the musical signal, it should deliver nothing but a pure torrent of energy to the speakers, without showing any coloration, hesitation, or limitation. It would be no exaggeration to say that such an amplifier should exist like a perfect mirror, reflecting only the truth of the input signal 1.
This rigorous ideal has been physically manifested as a block of metal in the Boulder Amplifiers 2150 Monaural Power Amplifier. This amplifier is not merely an audio component. It represents a philosophical proposition thrust into the modern high-end audio world by a manufacturer whose origins lie in the realm of professional recording—where “accuracy” exists not as personal preference but as an absolute prerequisite 2.
In this review, we will analyze this Boulder 2150—this solitary existence—from multiple angles, delving into its technical depths and the essence of the acoustic experience it delivers. This journey will shake our established notions of “musicality” and “fidelity,” confronting each of us with the fundamental question: What is the purpose of audio reproduction?
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Boulder 2150 Mono Power Amplifier — Overview
- Manufacturer: Boulder Amplifiers, Inc. 3
- Model: 2150 Mono Power Amplifier 4
- Release Period: Introduced to market between 2013-2017 as successor to the 2050, which was produced for over 17 years 5
- Price Range:
- Key Specifications:
- Peak Power Output: 1,000W (8Ω), 2,000W (4Ω), 4,000W (2Ω) 4
- Total Harmonic Distortion (THD, 1000W, 8Ω): 0.0008% (20Hz–2kHz), 0.0035% (20kHz) 4
- Frequency Response: +0.00, -0.04 dB (20Hz–20kHz); -3dB @ 0.015Hz, 200kHz 4
- Voltage Gain: 26 dB 4
- Input Impedance: 200kΩ (balanced) 4
- Output Impedance: Nominal 0Ω (zero feedback) 5. Stereophile measured value: 0.02Ω 14
- Weight: 99.8 kg/unit 4
- Power Connector: 32A IEC standard 4
1. Third-Party Perspectives: Intersection of Evaluations
No audio equipment can be fully understood from a single viewpoint. The evaluations surrounding the Boulder 2150 present an intriguing landscape where praise for its technical excellence intersects with vigorous debate over its sonic character.
| Media | Quoted Excerpts (Translation + Original) | Rating | Bias Analysis |
|---|---|---|---|
| Stereophile (M. Fremer) | “I felt the Boulders’ overall sound, while tonally neutral and utterly transparent, was somewhat dry.” 1 | ★★★★☆ | Highly reliable. Fremer is an experienced reviewer, and his “dry” criticism carries specific, nuanced meaning rather than simple negation. This is important criticism, not based on prejudice. |
| The Absolute Sound (A. Cordesman) | “This one of the least-colored, best-performing amplifiers ever made.” 15 | ★★★★★ | Extremely positive. Cordesman views its neutrality as an absolute good, evaluating that it reveals flaws in other system components. This represents a philosophical stance but is valid analysis. Likely manufacturer-loaned unit, but analysis is deep and aligns with Boulder’s design goals. |
| AudioShark Forum (user “bonzo”) | “Boulder is cleaner, faster, and can sound sterile if not matched properly.” 16 | ★★★★☆ | Experienced user perspective. Understanding the importance of system matching, recognizing both strengths (clean, fast) and potential weaknesses (sterile). Valuable real-world feedback uninfluenced by manufacturer. |
| YouTube (Jay’s Audio Lab) | “The number one amplifier that I have ever owned in my entire life.” 17 | ★★★★★ (98/100) | Enthusiastic owner review. While confirmation bias is possible, his passion provides a valid counterpoint to more analytical criticism. High evaluation based on performance in his system (Magico M6 speakers). |
Aggregating these evaluations reveals a clear consensus: the 2150 is a technical marvel with unparalleled control capability and neutrality. However, there’s a consistent subjective criticism—a sonic texture described as “dry” or “sterile.” This is not wholesale condemnation but rather a consistent observation that defines this amplifier’s character. Positive reviews praise its “accuracy” as the ultimate goal, while more critical reviews question whether this accuracy comes at the expense of musical soul.
This “sterile” versus “neutral” dichotomy may actually represent a deeper audio philosophy debate. Critics and users consistently describe the 2150’s sound with words like “neutral,” “transparent,” “clean,” and “fast” 1—objective descriptions of its technical behavior. However, some observers, including Michael Fremer, supplement this neutrality with subjective terms like “dry” and “lacking musical flow” 1. This is not contradiction but interpretation difference. What one person calls “perfectly neutral,” another experiences as “emotionally sterile.”
Therefore, the core of this controversy is not whether the amplifier is accurate—measurements prove its accuracy 14. The real question is: Is perfect accuracy the most desirable quality in music reproduction? The Boulder 2150 serves as a litmus test for audiophiles’ fundamental beliefs.
2. Design Philosophy: The Abyss of Uncompromising Engineering
To understand the Boulder 2150’s physical presence and acoustic characteristics, we must unravel the philosophy underlying its design philosophy—a kind of monastic pursuit of pure performance without any compromise.
Boulder’s DNA: From Studio to Audiophile Living Rooms
Boulder’s history originates in manufacturing professional broadcast and recording equipment 2. This heritage forms the foundation of their design philosophy. In recording studios, playback equipment should not add its own interpretation to music. It must be a “tool” that reproduces recorded sources exactly as they are, without any coloration. The legendary 2000 series, introduced in 1995, fully embodied this “uncompromising” philosophy, where performance determined design and cost was secondary 2. The 2150 is a direct descendant carrying this DNA.
Core Technologies: The Trinity of Control
The 2150’s overwhelming performance is supported by three interrelated core technologies:
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99H2 Gain Stage: This is not a commercial component. It’s a fully discrete, surface-mount modular operational amplifier developed in-house by Boulder, potted with resin for thermal stability and shielding 5. An evolution of the legendary 990 gain stage used in studios, its purpose is voltage amplification with extremely low noise and distortion, high slew rate, and wide bandwidth 5. The “H” in the model name signifies “High Voltage,” enabling operation with higher headroom and linearity 19.
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Active Class-A Bias Circuit: This technology symbolizes the 2150’s innovation. While conventional Class-A amplifiers operate at maximum bias constantly, generating enormous heat, the 2150 features intelligent analog circuitry that continuously monitors voltage, current, and load 5. When detecting steep transients in musical signals, it increases bias faster than the signal itself to maintain pure Class-A operation, then gradually reduces bias afterward 5. This achieves the sonic purity of Class-A operation (absence of crossover distortion) without the drawbacks of excessive heat or power consumption. Consequently, this massive amplifier can operate at relatively low temperatures 5.
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Aggressive Use of Negative Feedback (NFB): In the high-end audio world where “zero feedback” is often used as marketing rhetoric, Boulder’s philosophy stands apart. They assert that “properly designed and implemented feedback can achieve ideal operating parameters and constant group delay (maximally linear phase response) across the entire audio band” 5. According to technical documentation, feedback is precisely used for gain determination and bandwidth limiting, and its proper understanding and utilization represents the essence of Boulder design. They suggest that design philosophies avoiding feedback stem from outdated thinking originating from early slow operational amplifier limitations 21.
These design philosophies are strongly reflected in the physical structure. Heat sinks are machined in-house from approximately 36kg (80-pound) aluminum blocks using CNC machines 5. This isn’t merely for aesthetics—it’s to eliminate resonances that cause microphonic distortion and create a mechanically inert chassis 5. Eighty bipolar output transistors are pressure-fitted to heat sinks using CNC-machined bars rather than screws, ensuring excellent thermal conduction and reliability 5. This resembles products from military or aerospace industries more than audio equipment—a crystallization of what could be called obsessive engineering.
Boulder’s design philosophy doesn’t consist of independent technologies but forms a self-contained system where individual techniques organically combine toward a single purpose. The goal of absolute neutrality first demands the proprietary 99H2 gain stage achieving low noise and low distortion. Next, negative feedback is precisely introduced as a tool to ensure linearity and control across wide bandwidths. For this feedback loop to function stably and effectively, the gain stage itself must be extremely fast, which again justifies the 99H2’s existence. To supply energy to drive any speaker with this entire system, massive power supplies and numerous output devices become necessary. To operate this huge output stage with Class-A purity but realistic thermal management, the innovative active bias circuit was invented. Finally, to protect this sensitive, high-speed circuitry from mechanical vibrations that could become noise sources, a bank vault-like chassis is machined. Thus, one design choice inevitably leads to the next, with everything converging toward the single goal of “measurable perfection.”
Technical Comparison with Competitors
To better understand Boulder’s philosophy, let’s compare technical specifications with other ideological rivals reigning at the pinnacle of high-end amplification. This table is not mere number compilation—it’s a mirror reflecting different engineering philosophies held by world-class amplifier designers. Boulder’s astonishing low distortion, Dan D’Agostino’s overwhelming power, Pass Labs’ dedication to pure Class-A operation, and Gryphon’s combination of zero feedback with Class-A. These numbers provide objective foundations for subjective sound quality differences discussed later.
| Feature | Boulder 2150 | Dan D’Agostino Relentless Mono | Pass Labs Xs 300 | Gryphon Mephisto Solo |
|---|---|---|---|---|
| Output (8Ω/4Ω/2Ω) | 1000W / 2000W / 4000W | 1500W / 3000W / 6000W | 300W (Class-A) / 600W (Class-AB) | 200W (Class-A) / 400W / 800W |
| Total Harmonic Distortion (THD) | <0.0008% (1kHz, 1000W, 8Ω) | <0.007% (1kHz, 1500W, 8Ω) | 1% (at rated output) | <1% (at rated output) |
| Input Impedance | 200kΩ (balanced) | 100kΩ (balanced) | 200kΩ (balanced) | 10kΩ (balanced) |
| Damping Factor | >400 (estimated from 0.02Ω) | >267 | 200 | >400 (estimated from <0.025Ω) |
| Feedback Philosophy | Precise NFB utilization | Minimalist / auditory tuning | Minimalist / “small amount” | Zero global NFB |
| Weight (per chassis) | 99.8 kg | 258 kg | 61kg + 78kg (total 139kg) | 108 kg |
Source: 13
Measurement as Oracle: Dissecting Objective Performance
Proving that Boulder’s design philosophy crystallizes not as mere ideology but physical reality are the detailed measurements by John Atkinson published in Stereophile 14. These figures tell, with cold objectivity, why the 2150 possesses such dominant yet transparent acoustic characteristics.
- Output Impedance and Frequency Response: Measured output impedance, including speaker cables, was an astonishing 0.02–0.026Ω across the audible range 14—extremely close to the nominal “0Ω.” Consequently, frequency response changes due to speaker impedance variations are suppressed to “virtually non-existent” levels. This proves absolute control where the amplifier remains completely unaffected by speakers’ capricious behavior, simply driving signal relentlessly.
- Noise Floor: A-weighted S/N ratio recorded an excellent 97dB 14. More importantly is the “quality” of noise. Spectral analysis shows power supply-derived hum components below -110dB (0.0003%)—silence far beyond human hearing limits. This immeasurable quietness provides the objective foundation for the “pitch-black background” described in listening impressions.
- Distortion Characteristics: The 2150’s distortion is so low that under most conditions it’s buried in the measurement instrument’s noise floor 14. For example, even at high 400W output into 4Ω loads, distortion waveforms appear almost like noise, with spectral analysis barely revealing third harmonics at -119dB (0.0001%) 14. High-frequency intermodulation distortion was similarly concluded as “unusually low” 14. This is unshakeable evidence that Boulder’s philosophy of adding nothing and subtracting nothing from signals has been perfectly achieved.
- Output: Clipping (waveform saturation point) was measured at 960W into 8Ω and 1450W into 4Ω 14. While slightly below nominal values, this likely resulted from AC voltage drops during measurement, paradoxically demonstrating the enormous current this amplifier demands 14.
These measurements are not mere spec sheet numbers but oracles revealing physical laws behind subjective listening impressions. Absolute speaker control, pitch-black silence, and complete signal fidelity—all are no longer poetic expressions but objective facts proven by measurement in the Boulder 2150.
3. Sonic Exploration: Auditory Impressions
Technical theory gains meaning only when we understand how it manifests as auditory experience. Here we delve deeply into the central controversy surrounding the 2150’s sound quality—namely, its “character.”
| Critic / Media | Quoted Excerpts (Translation + Original) |
|---|---|
| Michael Fremer / Stereophile | ”It reproduced precise attacks and ear-popping decays, but the sustains of notes between those attacks and decays were less than fully expressed.” 1 |
| Anthony Cordesman / TAS | ”The Boulder 2150 has an almost incredible amount of reserve power and an extremely high damping factor.” 15 |
| HIFICLUB / YouTube | ”We were able to experience… the silky, liquid sound quality that is rare in a transistor amp.” 30 |
Genre-Based Analysis
Classical Music
The 2150’s strengths are fully displayed in this genre. Its immeasurable power and damping factor completely control large woofers, rendering timpani strikes in Mahler symphonies with terrifying impact and texture 5. The extremely low noise floor creates a pitch-black background from which micro-dynamics like page turns and violinists’ breathing emerge with surprising clarity 14. The soundstage is vast yet stable, with each instrument positioned in space with absolute precision 1. This might be better called a high-resolution acoustic imaging device rather than an amplifier.
Jazz & Acoustic Music
In this realm, debate over the 2150 intensifies. On one hand, its speed and accuracy are breathtaking. Pick attacks on steel strings, bass strings striking fingerboards, sharp transients of rimshots—all reproduced without any blurring or delay. However, Fremer’s criticism regarding “sustain” becomes significant here 1. The rich decay of piano chord resonances or warm, resonant saxophone tones might feel somewhat shortened or “less than fully expressed.” The fundamental is perfect, but the harmonic cloud that gives sound warmth and “life” might feel slightly thin. This is likely the source of the “dry” impression—hyper-realistic yet potentially lacking romanticism. Interestingly, HIFICLUB’s review mentions “silky, liquid sound quality” 30, suggesting that with proper systems (Kaiser Acoustics speakers), this amplifier can show different expressions, highlighting the importance of system matching.
Rock & Electronic Music
In these genres, the 2150 deserves to be called the ultimate weapon. Bass control is absolute. Stereophile’s review describes it gripping Wilson Audio Alexandria XLF woofers with “stronger clamp force” than reference amplifiers 5. Complexly layered synthesizer tracks are easily unraveled with each sound clearly separated. Its speed and “start-stop power” 5 convey music’s rhythm and pace as gut-shaking physical force. There’s no bloat or excess resonance—only pure, controlled energy.
4. Evaluation: Multi-Faceted Value Verification
To properly evaluate the Boulder 2150’s value, we must objectively analyze its performance across multiple axes.
| Evaluation Axis | Score (out of 5) | Commentary |
|---|---|---|
| Technical Performance | 5.0 | Measurements are literally state-of-the-art. Distortion approaching zero, massive power that doubles predictably, nearly zero output impedance, and extremely low noise floor. It represents an engineering benchmark that challenges even the finest measurement instruments 14. |
| Musical Appeal | 4.0 | This score reflects subjective evaluation divergence. For listeners prioritizing clarity, speed, and control above all, it would merit 5.0. However, consistent criticism regarding “dry” character and supposedly poor “sustain” expression 1 means it may not resonate with listeners preferring warmth and rich harmonics. It demands attention rather than seducing the listener. |
| Build Quality | 5.0 | Unparalleled. In-house CNC machining from aluminum blocks, resonance-eliminating interlocked chassis construction, and obsessive attention to detail transcend mere audio equipment, embodying the dignity of the finest industrial products 5. |
| Price-to-Value | 3.5 | ”Value” becomes complex at this price level. The price is astronomical 5. However, considering its performance, manufacturing quality, and that some competitors are even more expensive, value exists as the pinnacle of specific design philosophy. Boulder products’ long lifecycles also enhance long-term value 5. |
| Future-proofing / Repairability | 5.0 | Boulder products are known for very long product cycles (the previous 2050 was produced for 17 years) 5. Anecdotes about outstanding reliability are numerous 32. Corporate stability and complete in-house production suggest long-term support expectations. This possesses heirloom quality for future generations. |
Conclusion
Positive Aspects:
- Unparalleled technical measurements
- Absolute control over any speaker
- Infinite dynamic headroom
- Highest-level manufacturing quality and reliability
- Clear, consistent design philosophy
Considerations:
- Extremely expensive
- Massive weight and size requiring special installation
- Potentially “dry” or “analytical” sonic character
- Need for careful component selection to maximize system potential
In summary, the Boulder 2150 is not an amplifier for everyone, nor does it aspire to be. It represents the uncompromising execution of a unique vision: becoming a “completely transparent window with infinite power to recorded signals.” Its “shortcomings” are not engineering failures but subjective consequences of its objective success.
5. Panoramic Perspective: Coordinates in High-End Audio
By viewing the Boulder 2150 not as an isolated product but as coordinates on the philosophical map of high-end amplification, its true significance becomes clear. Modern pinnacle amplifier design can be classified into three major ideological currents.
First Current: The School of Purity Boulder represents the purest embodiment of this school. Their belief lies in the amplifier’s role as a “tool” for pure measurement and perfect fidelity. The ideal is “wire with gain,” where subjective “voicing” is considered a form of distortion. The 2150 demonstrates what becomes possible when engineering metrics are pursued to their logical conclusion—their masterpiece.
Second Current: The School of Artistry Pass Labs and Dan D’Agostino represent this current. While technical capability is essential, they believe the ultimate goal is emotionally appealing musical experience. Designers are artists giving “sound” to components. Nelson Pass speaks about choosing distortion “character” and the importance of simple circuits 33. Dan D’Agostino declares “I don’t care what it measures” and “I design for sound” 35. Their amplifiers approach “instruments” designed to create beauty.
Third Current: The School of Iconoclasm Gryphon Audio champions this current. They pursue different paths to acoustic truth by rejecting parts of established engineering orthodoxy, particularly global negative feedback 24. They achieve stability and control through brute-force elegance: massive, heavily-biased pure Class-A output stages and enormous power supplies.
Choosing among these amplifiers is not simply judging “good” or “bad”—it’s selecting a philosophy you deeply believe about audio reproduction’s purpose. The Boulder 2150’s value lies not only in its performance but in confronting us with the fundamental question: “Am I a scientist seeking objective truth, or an art lover seeking subjective beauty?“
6. Conclusion and Recommendations
The Boulder 2150 embodies one extreme of audio reproduction—a monumental product. It’s a tool of astonishing precision and power that completely dominates any speaker and strips away music signal’s final veils.
Recommended Users
- Analytical Listeners: Those wishing to hear every detail in recordings without any amplifier interpretation.
- Owners of Hard-to-Drive Speakers: Those with large Wilson Audio, Magico, Focal speakers demanding absolute control and infinite power 5.
- System Builders: Those wanting completely neutral, stable, reliable cores around which to voice systems with other components.
Users Requiring Careful Consideration
- Listeners Seeking Amplifier-Specific Warmth or Rich Coloration: This amplifier isn’t designed to add such character.
- Systems Already Bright or Analytical: The 2150 may further emphasize these tendencies, requiring careful matching.
- Physical/Budget Constraints: Its extreme price, weight, and size make it unrealistic for many audiophiles.
Future-proofing
This represents the completion of pure analog design. Moreover, “modifying” this amplifier would be sacrilegious, completely incompatible with its design philosophy and destructive to its integrity and value.
Overall Rating: ★★★★☆ (4.5/5)
The Boulder 2150 represents a great achievement in audio engineering. It may not be the most romantic or forgiving amplifier, but in its intended purpose—becoming an unwavering, uncorrupted conduit for musical signals—its performance is unparalleled. It’s closer to a “scientific instrument” for pursuing truth than a “musical instrument” for making music. For better or worse, it presents us with truth itself rather than interpretation.
References
1. Boulder Amplifiers 2150 monoblock power amplifier Page 2 - Stereophile.com, https://www.stereophile.com/content/boulder-amplifiers-2150-monoblock-power-amplifier-page-2 2. About Boulder Amplifiers - History - Boulder Amplifiers, https://boulderamp.com/about-boulder-amplifiers/ 3. Boulder Amplifiers 2150 monoblock power amplifier Specifications - Stereophile.com, https://www.stereophile.com/content/boulder-amplifiers-2150-monoblock-power-amplifier-specifications 4. 2150 Mono Power Amplifier - 2100 Series, https://boulderamp.com/products/2150-mono-power-amplifier/ 5. Boulder Amplifiers 2150 monoblock power amplifier | Stereophile.com, https://www.stereophile.com/content/boulder-amplifiers-2150-monoblock-power-amplifier 6. Boulder Debuts 2100 Series Components at 2013 Hong Kong High End Show, https://www.theabsolutesound.com/articles/boulder-debuts-2100-series-components-at-2013-hong-kong-high-end-show/ 7. Review of the the 2150 Mono Amplifier in Stereophile Magazine, https://boulderamp.com/review-2150-mono-amplifier-stereophile-magazine/ 8. Boulder Amplifiers 2150 Mono Power Amplifier | Music Lovers Audio, https://www.musicloversaudio.com/product-page/boulder-amplifiers-2150-mono-power-amplifier 9. Boulder Amplifiers, Preamplifiers, Phono, DACs | Music Lovers Audio, https://www.musicloversaudio.com/shop-boulder-amplifiers 10. Power Amplifiers - Reference Analog, https://www.referenceanalog.com/collections/power-amplifier 11. 【2023東京インターナショナルオーディオショウ速報】D棟5F アクシス／ヨシノトレーディング／ユキム - Stereo Sound ONLINE, https://online.stereosound.co.jp/_ct/17665024 12. 2150 MONO POWER AMPLIFIER / Boulder - U-AUDIO, https://www.u-audio.com/shopdetail/000000008467/ 13. Boulder 2150 Mono Amplifier - Reference Analog, https://www.referenceanalog.com/products/boulder-2150-mono-amplifier 14. Boulder Amplifiers 2150 monoblock power amplifier Measurements …, https://www.stereophile.com/content/boulder-amplifiers-2150-monoblock-power-amplifier-measurements 15. Boulder 2150 Mono Power Amplifier and 2110 Preamplifier - The Absolute Sound, https://www.theabsolutesound.com/articles/boulder-2150-mono-power-amplifier-and-2110-preamplifier/ 16. Boulder 2150 Stereophile review | AudioShark Forums - High End Audio, Stereo and Home Theater Systems Discussions, https://www.audioshark.org/threads/boulder-2150-stereophile-review.11264/ 17. Jay’s take on the Boulder 2150 monoblocks! - YouTube, https://www.youtube.com/watch?v=lhKf-g-Vs_A 18. Boulder Amplifiers - PS Audio, https://www.psaudio.com/blogs/copper/boulder-amplifiers 19. 2150 Tech Paper - Boulder Amplifiers, http://boulderamp.com/wp-content/uploads/2150-Tech-Paper.pdf 20. Boulder 2160 Stereo Amplifier - Reference Analog, https://www.referenceanalog.com/products/boulder-2060-stereo-amplifier 21. 3060 Tech Paper - Boulder Amplifiers, https://boulderamp.com/wp-content/uploads/3060-Tech-Paper-email.pdf 22. Untitled - Dan D’Agostino Master Audio Systems, https://dandagostino.com/files/sell-sheet-epic-1600-mono-amplifier_4652.pdf 23. D’AGOSTINO Relentless Mono - Audiohum, https://www.audiohum.com/gb/discontinued-product/d-agostino-relentless-mono 24. Mephisto - Gryphon Audio, https://gryphon-audio.dk/the-collection/power-amplifiers/mephisto/ 25. XS300 - Pass Labs, https://www.passlabs.com/products/xs300/ 26. Pass Labs XS 300 - USA Tube Audio, https://www.usatubeaudio.com/product/amplification/power-amps/pass-labs-xs-300/ 27. Xs 300 / Xs 150 Owner’s Manual - Pass Labs, https://www.passlabs.com/wp-content/uploads/2019/12/Xs-amp_om.pdf 28. Pass Labs XS300 - Ljudshopen, https://www.ljudshopen.se/produkt/pass-labs-xs300/ 29. Gryphon Mephisto Mono Power Amplifier Pair with StandArt Plinths Ex Demo - DB Hifi, https://www.dbhifi.co.uk/product/mephisto-mono-amplifier-with-plinths/ 30. [Audio Session] The Fateful Encounter of Boulder and KAWERO! | Part 2 - YouTube, https://www.youtube.com/watch?v=yM2UD2kzj9U 31. Boulder 1060 power amplifier | Stereophile.com, https://www.stereophile.com/content/boulder-1060-power-amplifier 32. Boulder + dCS (2160 review) - Audio Systems, https://dcs.community/t/boulder-dcs-2160-review/6482 33. INTERVIEW NELSON PASS OF PASS LABS AND … - Moremusic.nl, https://www.moremusic.nl/pass_labs/Brochures/interview-HifiPig_NelsonPass.pdf 34. Nelson Pass Interview - Audiophile Review, https://audiophilereview.com/amps/nelson-pass-interview/ 35. Dan D’Agostino - Positive Feedback, https://positive-feedback.com/interviews/dan-dagostino-interview/ 36. Heritage - Gryphon Audio, https://gryphon-audio.dk/heritage/ 37. Boulder 866 Integrated - Full Review : r/audiophile - Reddit, https://www.reddit.com/r/audiophile/comments/q6w2nu/boulder_866_integrated_full_review/ 38. The Focal—JM Lab Utopia III ‘Grande Utopia EM’ Loudspeaker - Audiophilia, https://www.audiophilia.com/reviews/2016/3/17/the-focal-jm-lab-utopia-iii-grande-utopia-em-loudspeaker