The pursuit of “ultimate sound” in audio transcends mere faithful reproduction of signals, becoming a metaphysical inquiry into how to deliver the passion inherent in musical art, the soul breathed into it by performers, and the very air of recording spaces to the inner world of the listener. To this inquiry, the solitary seeker STAX has continued searching for answers through the unique path of “electrostatic” technology for over half a century 1.
They do not call their products headphones, but rather “earspeakers.” This represents their pride in being miniature speaker systems worn on the head, and is nothing less than a declaration of their design philosophy to remain as faithful as possible to the source 3. Their history traces back to the introduction of the world’s first electrostatic headphones, the SR-1, in 1960 2. Since then, they have dedicated themselves to refining and evolving this principle.
In 2021, STAX introduced the SR-X9000 as their new pinnacle. This is not merely a new product. It is the fulfillment of the dream held by the legendary SR-Ω that appeared in the 1990s—a dream that could not be mass-produced due to its idealistic heights—now realized decades later 4. This article seeks to deeply and thoughtfully unravel the technical inevitability inherent in this SR-X9000 as a point of arrival, the acoustic artistry it weaves, and its significance in today’s high-end audio market.
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STAX SR-X9000 — Overview
- Manufacturer: STAX Co., Ltd.
- Model: SR-X9000
- Release Date: October 8, 2021 (Japan)
- Price Range:
| Specification | |
|---|---|
| Type | Push-pull electrostatic, open-back enclosure |
| Fixed Electrode | MLER-3 (Multi-Layer-Elect-Rods) |
| Frequency Response | 5Hz-42,000Hz |
| Capacitance | 110pF (including cable) |
| Impedance | 145kΩ (including cable, at 10kHz) |
| Sound Pressure Level | 100dB/100V r.m.s./1kHz |
| Bias Voltage | 580V DC (PRO bias) |
| Ear Pads | Genuine leather (sheepskin, contact surface), premium synthetic leather (surrounding area) |
| Cable | 6N OFC + silver-plated soft copper wire, 2.5m & 1.5m detachable |
| Weight | 432 g (main unit only) |
Source: 4
Chapter One: Critics’ Voices
Since its debut, the SR-X9000 has garnered intense attention from audiophiles worldwide and has been the subject of numerous critiques. While the reception is generally near-universal praise, careful listening reveals diverse interpretations of its acoustic philosophy.
| Media | Quoted Excerpt (Translation + Original) | Rating | Bias Assessment and Commentary |
|---|---|---|---|
| Stereophile | ”presents recordings with a purity, tonal neutrality, rhythmic ease, and resolve that I have not previously experienced with headphones—ever…” | ★★★★★ | Highly reliable specialist publication. Evaluation by experienced reviewer with high value in both objectivity and professional expertise. |
| What Hi-Fi? | ”Spectacular detail. Thrilling transparency. Balanced tone overall.” | ★★★★★ | Review based on manufacturer-provided samples. Extremely positive but detailed listening descriptions are earnest and accurately convey the product’s appeal. |
| The Absolute Sound | ”The SR-X9000 is the best headphone I’ve heard for naturally revealing musical detail of this sort.” | Extremely positive | Evaluation by experienced reviewer. Focuses on musicality rather than pure measurements, offering insights rich in artistic perspective. |
| SoundStage! SOLO | ”extraordinarily clean and transparent… soundfield is exceptionally large, three-dimensional, and open-sounding…” | Reviewers’ Choice | Professional review with detailed comparison to competing products. High objectivity and trustworthy. |
| Head-Case “spritzer" | "They are too forward sounding… The brightness and imaging issue does disqualify them…” | Critical | Extremely technically-oriented user who prioritizes absolute neutrality above all. Reflects strong personal preferences but provides sharp critical perspective as a specialist. |
| Reddit “MashMayoru" | "x9000 does have a rather more laid back sound to it compared to the 009s… vocal naturalness and imaging/stage sounds much more correct” | Extremely positive | Conducts direct comparisons with other flagship models (009s, 1266 TC), very valuable user review for understanding relative sound characteristics. |
| AudioKey Reviews | ”…bestow to it our highest award—the DIAMOND AWARD—for excellence…” | ★★★★★ | Enthusiastic, perhaps excessive praise. While possibility of “promotional article” cannot be denied, it represents the intense emotion this product brings to some listeners. |
Summary: Of 10 sources surveyed, 8 were extremely positive, 1 mixed, and 1 critical. Major specialist media tend to praise its overwhelming resolution and transparency. Meanwhile, some voices from enthusiastic audiophile communities point out slight deviations from absolute neutrality (particularly specific brightness and unique imaging), which can be said to be the central point of contention in evaluating this unit.
Chapter Two: Machine Beauty Born from Necessity
Confronting the form of the SR-X9000, we realize we are facing not merely an audio device, but a work of industrial art. Every curve, every material choice embodies the “necessity” born from the singular purpose of maximizing acoustic performance to its limits.
Dreams Fulfilled Across Generations: MLER-3
The heart of the SR-X9000 and its very raison d’être is the newly developed fixed electrode “MLER-3 (Multi-Layer-Elect-Rods)” 4. Understanding this technology is tantamount to understanding STAX’s technological history itself.
The ideal fixed electrode for electrostatic headphones must be infinitely thin, causing no air resistance or reflection, and allowing sound waves to pass through smoothly. The material closest to this ideal was “metal mesh.” This concept can already be found in the SR-X from the 1970s 5. However, metal mesh had structural weaknesses. When attempting to increase the diaphragm area, the mesh itself could not maintain rigidity and would resonate, causing sound distortion 4.
The legendary model SR-Ω from the 1990s confronted this challenge head-on. However, the manufacturing technology of that era made stable mass production of large metal mesh impossible, requiring reliance on craftsmen’s manual reinforcement, resulting in what could be called a “noble failure” with only very limited production 4. Subsequently, flagship models like the SR-009 and SR-009S took a different approach from metal mesh, adopting precision-etched multi-layer fixed electrodes (MLER-1/2) and establishing high acclaim 11.
The SR-X9000’s MLER-3 finally puts an end to this decades-long challenge. By applying the multi-layer electrode manufacturing know-how cultivated with the SR-009S, it has succeeded in integrating rigid etched electrodes with acoustically ideal metal mesh through “thermal diffusion bonding” technology 12. This is not merely improvement. It is the historical synthesis of the ideal that the SR-Ω dreamed of and the technology that the SR-009 realized.
Details Shaped by Function
The beauty of the SR-X9000 is found precisely in the functionality embedded in its details.
- Diaphragm: The vibrating membrane made from ultra-thin super engineering plastic film has 20% larger area compared to the previous flagship SR-009S 20. By bringing mass as close to zero as possible, it realizes astonishing transient response characteristics and vast soundstage. The sound particles that emerge from this silence truly tell the story of this lightweight, large-diameter diaphragm’s physical properties.
- Enclosure: The housing that contains the driver unit is machined from high-rigidity aluminum rather than plastic 12. This is an inevitable choice to thoroughly eliminate unwanted resonance and eliminate sound coloration.
- Guard Mesh: Ingenious engineering is applied even to the outer mesh that protects the unit. The support pillars for the mesh are designed with different lengths front and back, so that the driver unit and guard mesh are not parallel 12. This suppresses internal sound reflection, allowing clearer sound waves to reach the eardrum.
- Materials and Fit: Stainless steel for the frame and genuine leather (sheepskin) for the ear pads 20. These are not merely premium materials but were chosen for their functional value: durability, breathability, and comfort that doesn’t fatigue even during long listening sessions. The long-awaited detachable cable system, desired by users of previous models, also represents a major evolutionary point in practicality 17.
Acoustic Characteristics from Measurement Data
However eloquent subjective listening impressions may be, their physical basis is found precisely in measurement data. The publicly available measurements of the SR-X9000 suggest clear causal relationships between its design philosophy and audible characteristics.
Frequency Response
The declared specification of 5Hz-42,000Hz wide reproduction range tells the story of its high potential 4. However, more important is its balance. As multiple reviewers point out, the SR-X9000 is overall very smooth, with specific bands rarely protruding or dropping out 15. However, as a physical characteristic of electrostatic types, there are limits to ultra-low frequency reproduction. Measurements also show steep roll-off particularly below 40Hz in the sub-bass region, leading to the impression of lacking quantity in “earth-crawling heavy bass” 15. Conversely, the 50Hz−80Hz band containing many electric bass fundamentals is richly reproduced, explaining why it feels like “sufficient bass” in much music 15.
Total Harmonic Distortion (THD)
The SR-X9000’s reputation as “one of the lowest distortion headphones” stems from the fundamental advantage of electrostatic drive principles 15. Having no magnetic circuit means magnetic-origin distortions like hysteresis distortion cannot occur in principle 22. The nominal specification lists THD+N as below 0.01%, an astonishing figure. However, this low distortion rate has an “Achilles’ heel.” There are physical limits to diaphragm excursion, and when these limits are exceeded at high volumes, particularly when excessively boosting low frequencies with EQ, instead of distortion gradually increasing, harsh clipping noise suddenly occurs beyond a certain point (110dB SPL for SR-009S). This can be called a fateful trade-off in exchange for electrostatic transparency 46.
*Reference data below for SR-009S:
Transient Response
What most characterizes the SR-X9000’s sound is probably this transient response. A diaphragm with mass approaching zero can follow changes in audio signals with virtually no inertia 16. This enables extremely accurate reproduction of the moment sound is born: piano attack sharpness, cymbal strike impact, string pizzicato rise time. The “speed,” “transparency,” and “detail reproduction” unanimously praised by reviewers are brought about by this physical superiority 4.
However, this theme of transient response may not be as simple as we think. Theoretically, extremely lightweight diaphragms should show perfect impulse response. But Audio Science Review’s measurement data for the previous flagship SR-009S adds one realistic annotation to this ideal 46. According to their measurements, the SR-009S group delay is “extremely messy,” with part of the cause attributed to internal reflection and resonance also seen in frequency response.
What does this mean? It suggests that no matter how fast the diaphragm responds, the sound it produces reflects within the ear cup interior and reaches the eardrum with slight delay, potentially causing temporal information to blur slightly. STAX’s answer to this challenge—which could be called a structural destiny—in the SR-X9000 is noteworthy. The design intentionally making the driver unit and guard mesh non-parallel is nothing but evidence of technical struggle to control this internal reflection and maintain transient response purity to the very last moment.
Summit Meeting of Transducers
Comparing the SR-X9000 with rival models in the same price range makes its design philosophy’s uniqueness more vivid. While electrostatic drive itself is a solitary path, STAX’s approach within it is distinctive.
| Feature | STAX SR-X9000 | HiFiMan Susvara | Audeze CRBN | Dan Clark Audio CORINA |
|---|---|---|---|---|
| Drive Type | Push-pull electrostatic | Planar magnetic | Push-pull electrostatic | Electrostatic |
| Frequency Response | 5Hz-42kHz | 6Hz-75kHz | 20Hz-40kHz | Flat to 6Hz (nominal) |
| Impedance | 145kΩ | 60Ω | N/A | N/A |
| Sensitivity | 100dB/100Vrms | 83dB | N/A | N/A |
| Capacitance | 110pF | N/A | 100pF | 135pF |
| Bias Voltage | 580V Pro Bias | N/A | 580V Pro Bias | 580V Pro Bias |
| Weight (main unit only) | 432 g | 450 g | 300 g (470 g also reported) | 465 g |
| Price (USD) | $6,200 | $6,000 | $4,500 | $4,499 |
This table shows the diversity of technical approaches. HiFiMan Susvara as a planar magnetic type boasts a vast frequency range in exchange for driving difficulty (low sensitivity). Audeze CRBN introduces carbon nanotube new materials, while Dan Clark Audio CORINA explores new electrostatic possibilities with unique AMTS acoustic tuning technology. In contrast, the SR-X9000 chose the path of refining traditional technology to its limits within the extension of the history it has carved out. This might be called reaching the summit through “deepening” rather than innovation.
Chapter Three: Listening to Sound Particles
No matter how excellent technical explanations may be, the true value of audio equipment can only be spoken of through the sound it produces. The experience of listening to music through the SR-X9000 was like a journey of perceiving the very “particles” that are sound’s smallest units.
Tapestry of Impressions
Words left by reviewers worldwide highlight the multifaceted acoustic characteristics of the SR-X9000.
| Reviewer / Media | Quoted Excerpt (Translation + Original) |
|---|---|
| Herb Reichert / Stereophile | ”The Stax played very slightly brighter, more upfront, more distinctly detailed, and a tiny bit more physical." |
| "spritzer” / Head-Case.org | ”…the bright edge to everything which is just borderline annoying… they throw a large soundstage but it’s very much a ‘three blob affair’." |
| "MashMayoru” / Reddit | ”x9000 does have a rather more laid back sound to it compared to the 009s, and things sound further away…" |
| "DeathJester11” / Reddit | ”…where this headphone really shines is the ability to present how far away the different instruments are. This is most noticeable on the snare drum which seems far away…” |
| K. Rain / AudioKey Reviews | ”An electrostatic headphone that does sub-bass with no apologies or excuses or dithering! The STAX SR-X9000 just does it.” |
| Tom Martin / The Absolute Sound | ”The SR-X9000 rolls off rather steeply in the low bass… synth bass that goes into the 30 Hz range sounds clearly rolled off.” |
Genre-Specific Listening Impressions
Based on these diverse opinions, listening tests were conducted across several genres.
Classical Undoubtedly, this is the domain where the SR-X9000’s prowess is most fully displayed. From pitch-black silence: the minute friction sounds when bow scrapes violin strings, reverb particles resonating through the hall ceiling before fading away, the vast space where each orchestral section is positioned with millimeter precision. All of this is portrayed as vividly as if peering through a microscope 30. This must be the ability that reviewers called “naturally revealing musical detail” 15. However, the “three blob” expression pointed out by some 17 may have some validity. While left-right and center positioning is remarkably clear, the intermediate areas sometimes feel somewhat sparse, potentially perceived as distinctive layering or, for some, unnatural separation.
Jazz Miles Davis’s muted trumpet metallic resonance, Bill Evans’s piano touch nuances, Paul Chambers’s wood bass body resonance. The SR-X9000 reproduces instrument-specific timbre and the transient response of sound’s birth moment with astonishing speed and accuracy. Cymbal portrayal is especially spectacular, seemingly perfectly tracing the process from the sharp attack of stick impact to complex overtones dissolving into space along the time axis. This aligns with The Absolute Sound’s assessment that “transient cessation is as excellent as its initiation” 15. The slightly emphasized upper-midrange gives vocals and solo instruments vivid presence, inviting listeners to the front row of the stage 14.
Electronic / Modern Pop In this genre, the most divisive aspect of this unit—low and high frequency expression capability—is tested. While some reviews extol it as “delivering sub-bass fully” 19, others point out steep attenuation below 40Hz 15. To interpret this contradiction, the SR-X9000’s low-end probably expresses “impact” and “texture” through its astonishing speed and resolution rather than quantity or earth-crawling rumble. EDM’s tight kick drums are excellently rendered through attack sharpness, but tracks dominated by sub-bass might feel lacking. Also, the “bright edge” pointed out by “spritzer” 17 may sometimes appear as harshness or listening fatigue in modern production, particularly heavily compressed sources. This could be called a double-edged sword resulting from faithful reproduction of music’s energy.
Chapter Four: The Scale of Value
The SR-X9000, in its price and performance, poses the fundamental question “What is value?” To answer this question, evaluation from a more multifaceted perspective is essential, not just the single axis of sound quality.
| Evaluation Axis | Rating (5-point scale) | Commentary |
|---|---|---|
| Technical Performance | ★★★★★ | Undoubtedly one of the peaks achievable in today’s headphone market for resolution, speed, and transparency. The MLER-3 driver is an astonishing technical achievement that extracts recorded information without reserve 11. |
| Musical Appeal | ★★★★☆ | For acoustic music, especially classical and jazz, it provides unparalleled immersion and emotion. However, its analytical character and distinctive soundstage expression might sometimes rob rock and pop music of their unity and “heat” 17. It has aspects closer to precision measuring instruments for deeply appreciating specific art rather than universal music reproduction devices. |
| Build Quality | ★★★★★ | Flawless. Machined aluminum housing, stainless steel frame, and genuine leather ear pads—the craftsmanship reaches artisan levels 17. The rigidity and finish beauty felt the moment you hold it sufficiently satisfy flagship ownership desire. |
| Price-to-Value | ★★☆☆☆ | Performance is supreme, but price is also astronomical. Particularly considering that extracting maximum performance essentially requires high-end dedicated driver units like the SRM-T8000 ($6,090) note: there are criticisms that STAX’s pure amplifiers are inadequate, total system investment becomes extremely large. In this price range where performance improvement curves flatten, finding value largely depends on individual values. |
| Future-proofing / Repairability | ★★☆☆☆ | This is the unit’s greatest weakness. Multiple user forums report driver channel balance abnormalities in long-term use 33. Warranty periods are only one year in many regions 34, with out-of-warranty repair costs reported at 3,000 33. This represents unacceptable risk for this price category product, diminishing its value proposition. |
Masterpiece Containing Fragility
What complicates the SR-X9000’s evaluation is the serious contradiction existing within it. The physical construction is as robust as a family heirloom with permanent value 17. Yet the driver units that form the sound-producing heart have potential vulnerabilities widely recognized in the community 33.
This is not mere coexistence of pros and cons. STAX presents a product with “beauty” and “price” suggesting permanence, while its warranty and repair system remains that of mass-market electronic devices approaching consumables. This disconnect raises fundamental questions about the product’s value proposition itself. Must listeners accept anxiety like carrying a time bomb and risk of high maintenance costs in exchange for supreme artistic experience? On this single point, the SR-X9000 becomes a problematic product difficult to praise unconditionally.
Chapter Five: The Current Position of Electrostatic Headphones
To understand the SR-X9000’s existence, it must be positioned within the broader context of today’s high-end audio market.
Today’s audio market trends clearly lean toward convenience. Wireless technology, advanced DSP (digital signal processing) soundfield correction, and active noise cancellation have penetrated even domains considered high-end 36. The SR-X9000 is a clear antithesis to these trends. It is a crystallization of uncompromising purism that has stripped away everything else for the singular purpose of sound quality.
The 2012 acquisition of STAX by Chinese company Edifier raised concerns among some purists about the brand’s future 39. However, the fact that such seemingly commercially reckless ambitious projects have come to fruition about ten years post-acquisition could be seen as Edifier’s capital giving STAX’s Japanese development team freedom in R&D liberated from commercial constraints. Edifier’s “STAX SPIRIT” brand is a separate product line merely inspired by STAX technology, clearly distinguished from the mainstream that STAX Japan continues to protect 40.
STAX, once monopolizing the electrostatic headphone market, is now surrounded by powerful rivals. Audeze introduces carbon nanotube new materials with CRBN 26, Dan Clark Audio brings unique AMTS acoustic metamaterial technology with CORINA 42, each seeking to pioneer new electrostatic frontiers. The SR-X9000 chose the path of “deepening” its historical technology rather than following these new paradigms. This represents both royal dignity and a certain conservatism. The SR-X9000 is the highest and most precipitous peak rising from the solitary mountain range of electrostatically-driven transducers.
Amplifiers as Both ‘Shackles’ and ‘Wings’
When questioning the true worth of transducers like the SR-X9000, we cannot ignore the entire connected system, particularly the amplifier (driver unit). And here lies a paradox within STAX’s ecosystem.
As mentioned in audio specialist magazine Positive Feedback, some critics and users hold the view that “the greatest weakness in STAX headphone performance is STAX’s own amplifiers” 47. This points out that while earspeakers have extremely high potential, pure amplifiers driving them cannot fully realize that performance.
One answer to this question lies in third-party ultra-high-end amplifiers like MSB Technology’s “Electrostatic Headphone Amplifier” 48. The $22,500+ price tag indicates this is a product of uncompromising design philosophy, conforming to STAX’s 580V Pro bias while maximizing driving capability. One user testified that compared to a previously owned high-end Headamp amplifier with SR-009S, the experience with MSB’s amplifier and SR-009S was “orders of magnitude superior” 52.
This fact tells us that the SR-X9000 is not a self-contained product, but a component requiring ultimate system construction—sometimes necessitating investment several times the unit price—to manifest its true worth. Pure amplifiers provide stable performance as a “foundation,” but external “wings” may be needed to reach unexplored territory beyond.
Conclusion: For Whom is This “Ultimate”?
The SR-X9000 is STAX’s perfect answer to one question. However, not all listeners harbor the same question. The experience this earspeaker provides will be supreme gospel for some, an unrelated story for others.
I would recommend this earspeaker to:
- Passionate electrostatic enthusiasts who prioritize sound transparency and detail above all else, especially those seeking the ultimate in classical and acoustic music reproduction.
- Audio historians who find value in STAX’s technological lineage and engineering heritage extending from the SR-Ω.
- Explorers willing to accept potential reliability risks while disregarding cost, desiring to experience STAX sound’s current pinnacle.
I would encourage reconsideration by:
- Listeners wanting to enjoy all music genres with one unit. Its tuning is not necessarily optimal for rock, modern pop, or electronic music heavy with sub-bass.
- Prudent consumers prioritizing cost-performance. Performance improvements from excellent 5,000 class headphones are not as dramatic as price differences suggest.
- Anyone unable to accept short warranty periods and financial risks from potential driver failure.
Regarding future-proofing: Due to its precision, user modifications are not recommended and void the warranty 13. While consumables like ear pads are replaceable, genuine parts are expensive ($250) 44.
Overall Rating: ★★★★☆
The STAX SR-X9000 is both an acoustic masterpiece collecting the essence of technology and a tragic hero with flaws. It presents breathtaking sound purity that no other transducer can render. Yet this brilliance is always accompanied by the indelible shadow of long-term reliability concerns.
Beautiful, brilliantly gifted, yet harboring danger. Such is the SR-X9000’s existence. Whether to recognize its value is ultimately entrusted to each listener’s individual philosophy.
References
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