Best Headphones Series: Stax SR-X9000 electrostatic headphones

Sound Quality

Theoretical audio engineering is fine, but really what we care about is sound quality. So, we applied our objective observational methodology to listening to the SR-X9000. For those of you confused or concerned about the combination of the terms “objective” and “observational” it may be worth reading our review methodology paper. A link is in the description. In a nutshell, we endeavor to describe the sonic things a device under test does, with reference to the sound of real instruments in real spaces. We endeavor to skip our opinions of whether we “like” the sound. Since every device has distortions, you have to judge whether the particular distortion profile of a device suits your needs (which may vary with your sensitivities, musical preferences and environment).

This is particularly true for a product like the SR-X9000 because I’m going to have to explain that it is impressively low distortion within its operating band. If you assume that is a subjective view, you’ll miss two key points. One is that the SR-X9000 is among the lowest distortion headphones available. The second point you might miss, and which may help you understand the objective nature of that observation, is that the SR-X9000 has a sonic profile that will not appeal to all users.

Voicing

We start with voicing, or the frequency response and octave-to-octave balance of the SR-X9000. Let’s run down a few details before covering the subtleties:

• Bass and treble are impressively balanced. The SR-X 9000 doesn’t immediately sound bass shy or bass heavy, nor is it treble-bright or treble-dark.
• Overall, the SR-X9000 sounds very smooth; there just aren’t frequency ranges that seem peaky, nor are there ranges with suckouts. You hear this as you listen to a range of instruments that operate up and down the spectrum and each of them sounds open and clear, something that doesn’t happen with big peaks and dips in the curve.
• If anything, the SR-X9000 has a rather flat response from mid-bass to mid-treble, which many listeners will hear as slightly emphasizing upper midrange because research shows most listeners want a response that falls off smoothly from the bass to the treble by several db. More on this in a bit. Note that almost all headphones have more significant deviations from ideal.
• The SR-X9000 rolls off rather steeply in the low bass. To square this with my comments on mid-bass, consider that Blu de Tiger’s “Enough 4 U”, which features her Fender Jazz electric bass, sounds rich and punchy and detailed. A bass guitar is tuned to a lowest frequency of 41 Hz and much of what we hear as foundational is between 50 and 80 Hz. That range is well done on the Stax. But Francine 13’s “Queen Mary”, which has synth bass that goes into the 30 Hz range sounds clearly rolled off. You may care, you may not, since the below 40 Hz range is very music dependent.
• The lower midrange seems to be very slightly recessed. At times this means vocals lack a bit of fullness. On the other hand, the lower midrange and upper bass work together nicely. I thought the cellos and basses on the Shostakovich Piano Concerto No. 1, with Yuja Wang and the BSO had the presence that you hear in live performances. I made a related note on Stereolab’s Instant Holograms on Metal Film.

Clarity and Transparency

Frequency response isn’t everything by a long shot. The SR-X9000, does in my observation have excellent frequency balance in its range when measured against the absolute sound. But it does go beyond this, which is where its secret sauce lies for some listeners.

The SR-X9000 is exceptional in my tests for its ability to unearth or resolve low level details. This might sound like some kind of BS, musically irrelevant preoccupation with minor things. But if you love music, it is anything but. When instruments play, they resonate. The resonances create harmonics (multiples of the basic frequency being played) and those harmonic signatures are how we know it is a violin vs a cello being played. And how we hear how beautifully or passionately the instrument is being played. And these harmonics become lower and lower in level over time and as they reflect in the performance venue.

The SR-X9000 is the best headphone I’ve heard for naturally revealing musical detail of this sort. Instruments and voices on the SR-X9000 just come alive, with an air and tonality that is convincing and thus beguiling.

At the same time, the SR-X9000 is special in this department because it doesn’t over-emphasize treble or other frequency ranges to trick you into thinking it is “revealing”. It sounds like it actually can reproduce very small signals.

Dynamic Capability

And large signals. Because the SR-X9000 also excels on difficult transients like rim shots and cymbal strikes and trumpet blasts. The excellence here, like the excellence with low-level sounds, seems due to accurate signal handling, not distortion. You sense this because of the ability of the SR-X9000 to stop a transient as masterfully as it starts one. I heard this on the drums of the Hadouk Trio’s Air Hadouk and on the piano of Keith Jarrett’s million notes per second New Vienna.

Now, I do think the midrange recess that I mentioned earlier means that some power music is not done perfectly. Power music tends to rise and fall, swell and recede, not instantly but still quickly as the whole band turns up the wick. This kind of dynamic swing works fine on the SRX-9000, but it is not exceptional here.

Imperfect Recordings

Many recordings are flawed. Some of these just make you wonder what the engineers were thinking. In any event, you might like your headphones to deal with this. It is of course hard to be faithful to the signal but moderate the bad signals. Oddly, I find that the very low distortion audio devices that come through the lab often do a reasonable job here. The SR-X9000 is like this. Edgy and bright recordings are edgy and bright, yes, but because of the apparently low distortion, the recording errors aren’t made worse. Some headphones that nominally might seem to mute the bad recording into submission still get triggered into ugly behavior. Sure, rolled off treble rolls off the uglies, but it isn’t a complete solution. If you listen to a lot of modern pop or 1970s and 1980s rock or classical or late ‘50s and ‘60s jazz, the SR-X9000 is worth hearing. You’ll hear the sonic warts but they may not bother you that much.

Of course, you can experiment with amplifiers to address this issue as well. The less expensive amps I had – the HiFiMan and the Stax solid state SRM-700S — revealed most of what the SR-X9000 does. That said I do think the SR-X9000’s full exceptional qualities benefit from one of the top amps. I didn’t have one on hand, but when I reviewed the Stax SR-009, I thought the Headamp Blue Hawaii amplifier romanticized the Stax sound a bit without overwhelming it’s resolution. That said, I did use the Ray Samuels B-21 and it was so low in distortion that I reconsidered my assumption that amelioration was needed for less optimal recordings. The B-21 made me see why listeners might want “Max Stax”, which it seems to deliver. Of course, you might want amp tuning just for artistic beauty. Your call.

Summary

The Stax SR-X9000 is a superb headphone for listeners who want to hear what is on the recording and love hearing the full detail of the instrument, the environment and the playing. It is a very engaging headphone, and not everyone wants that, but music lovers will be thrilled.