Background

The STAX SR-007 is the middle child of the STAX Omega family, first released in 1998, with subsequent revisions throughout 1999-2007. Of the original SR-007mk1, there are earlier revisions with 70XXX/71XXX serial numbers, and later revisions with SZ1-XXXX serial numbers (which I currently own). The original SR-007mk1 came in a very handsome champagne color and black or brown leather; the later versions only exist in silver/black and black/black combinations. It was released with the top-of-the-line SRM-007t amplifier, a fairly large vacuum tube solid-state hybrid energizer.

from the left: original Omega, SR-007mk1, SR-009

The STAX Omega family consists largely of the original Omega, the SR-007 "Omega II," and the latest SR-009/s, with a fourth member rumored to be in development at the time of writing. Compared to STAX's usual fare of boxy, retro-styled Lambda headphones, the Omegas are much more modern and sleek, with circular cups and (for the later Omegas) fine aluminum housing. They have all been considered the top of the line STAX offering when released, and depending on who you ask, each has had it's claim to fame as one of the best pair of headphones money can buy. While they are all fairly demanding headphones, the SR-007 is especially so, with 170kΩ impedance and extremely high current requirements. The common sentiment among STAX enthusiasts seems to be that the bare minimum requirement for these phones is the aftermarket KGSSHV design or a similarly-rated speaker amplifier (with an accompanying SRD-7mk2).

This particular pair of Omega IIs came from Japan, bundled with an accompanying SRM-007t for about $1,500 shipped ($200 of which was the shipping cost alone). I was very pleased to find that everything was in perfect working condition, and the main issue with the headphones was really not much of an issue at all: the cabling at the entry point into the phones was a bit finicky, and could cut out intermittently if not bent quite right or pulled. Given that the "stress relief" at the point of entry does nothing but exacerbate the issue, it didn't bother me and I found it to actually be much less of an issue than I thought. It's a fairly common issue with the SR-007mk1, as I understand it, and recabling it could prove disastrous due to the high sensitivity of the drivers, but fortunately it works perfectly fine otherwise.

Incidentally, the SRM-007t was itself a small rarity: while the vast majority of them are rated for and use 6CG7/6FQ7 tubes, this one came with vintage General Electric 8CG7/8FQ7 tubes, and has had the voltage tuned accordingly. It seems there were a small number of them made with a small batch of these quad-matched tubes.

Build & Aesthetics

The SR-007mk1 is, in my opinion, the most handsome of the STAX Omegas (actually, of all of STAX's offerings) and one of my favorite headphone designs period, though sadly not one you can bring around. The aluminum finish is extremely high quality and very durable, and the warm balance between the champagne metal and chocolate brown leather is quite classy. It fits quite well, with a much better seal than any of the Lambdas I've tried, and is quite comfortable for long listening sessions, though I will note the pressure on the glasses frames can become uncomfortable after a while (for me, around 3 hours of straight listening). It doesn't have that issue whatsoever without glasses. The stock pads have an interior curved "D" shape not unlike that of the Sennheiser HD800, and are angled a fair bit. I personally have the thicker side toward the back of my head, which I find gives me the best seal.

Easily the worst part of the build on the SR-007mk1 specifically is the strain relief (or lack thereof) on the connectors on each side. As you can see, there's actually little to no flexibility, which causes quite a bit of strain at the entrypoint. This issue is fairly widespread and known to be an issue on these early mk1 headphones, and while possible to recable and/or refit with the updated SR-007mk2 cables, it's quite difficult and potentially damaging due to the high heat involved.

As an aside: unlike pretty much any other headphone I'm aware of, for some strange reason, this has both rotating pads and cups. While I've grown used to it and now find it to be surprisingly nice for getting that proper seal, it was a rather strange chore to figure out when I first tried it on, and it's quite a pain to fit with new pads due to the two rotating pieces on each side.

Sound

The first thing I will say is that of the STAX headphones I've tried, this is the clear outlier—a feeling shared by many. Tonally, it runs a bit warm-neutral, and has a very authoritative bass response, with that coveted slam more commonly found on planar magnetic and closed-back headphones. The midrange tuning is much more natural, with less of a rise going into the lower treble regions than the STAX Lambdas. This upper midrange hump which seems to plague nearly all electrostatics is, to me, least pronounced in the SR-007, whereas it is quite apparent on the SR-λ, adding a certain sharpness or graininess to violin strings. The slightly recessed lower mids on the Lambda further contributes to a bit of an odd, thin timbre to piano notes as well, which isn't nearly as noticeable on the SR-007.

The bass response is the most remarkable to me, with great subbass extension and the sort of authoritative slam that one expects more from closed-back headphones. Even without any EQ, the physicality of drums and and cellos is made abundantly clear. The bass response was also the thing I found most improved upgrading from the rather weak SRM-252S to the KGSSHV Carbon: the extension seemed noticeably improved, down to 25Hz, and the details of the bass transients were (subjectively) much clearer than before.

I found the midrange and treble to be very rich, with enough air to give a sense of stage and atmosphere, but not to the point of unnaturally sharp details. Upper air past 16kHz and the transients of cymbals seemed to be somewhat improved by a stronger energizer here, but to a lesser degree than the improvement in the bass response. This upper treble sparkle, combined with the speed of the drivers, produces an almost ethereal presentation of live music, and is the most overt indication that the SR-007mk1 is still, after all, an electrostat at heart.

As with most open-back electrostatic headphones, the SR-007 has incredible staging and imaging capabilities. The stage is a bit more intimate than other STAX headphones I've listened to, but the imaging is remarkably clear with very accurate directional cues and depth. While the SR-λ stages like a small concert hall, the SR-007 sounds much more like a private venue or house band. I suspect this is partially due to the subbass extension's effect on room reflections.

Closing Thoughts

While this may not be the most STAX-like electrostatic headphone ever, it is certainly my favorite, both in aesthetics and sound. I do find some of the build design to be... questionable at best, but it's otherwise built very well and looks quite handsome. It's currently my daily driver, powered by a KGSSHV Carbon built by Søren Brix (sorenb), so I'm fairly certain I'm not missing out on any hidden technicalities by underpowering it.

In terms of value, depending on your perspective, I think this may actually be one of the better value headphones on the market; going by the common sentiment on the STAX Omega line compared to other TOTL headphones, the SR-007 is among the most affordable, well under half of the SR-009/s, Focal Utopia, or Hifiman Susvara. While it's true you will have to shell out for an accompanying electrostatic energizer, you'll likely still end up cheaper or on par with the high-end headphones and a matching amplifier. It's also worth noting that many of those high-end headphones are also commonly recommended to go with rather beefy amps; the Susvara, for example, is highly regarded but only when driven out of a speaker amplifier, which will run you an addition couple grand. With that being said, it's completely contingent on whether you do buy that the SR-007 truly competes with the best.

Finally, on the subject of how much the sound is influenced by different energizers, especially given the demanding requirements for this headphone in particular, I found that while there were definitely differences, I still think the overall sound is captured even when underpowered by something as small as the SRM-252s. That being said, some details—especially transients and staging—are definitely much improved with the KGSSHV Carbon. I certainly don't think you need such an expensive energizer to enjoy these headphones, however, but different people value different things and there are those who put much greater importance on the resolving ability of the headphone.

Personally though, I have the KGSSHV Carbon because it's nice and hefty. Placebo gains are still gains.