Stax SR-007 "Omega II" Open-Back Electrostatic Earspeaker

I'd straight with the bullet point. My review is based on my ears and personal experiences.

GEAR: Fiio K5 Pro -> 100W Class D Speaker Amp -> Stax Srd-7 Pro -> Can

Before EQ
Pros
- The female vocal is Amazing (Coherent and feel 3D)
- Super Details retrieval, speed, transparent
- The best bass for electrostatics (both quantity and quality)
- Very nice bass slam for e-stats
- Superb imaging like a laser

Cons
- Sound muddy due to warm/dark signature
- Stax fart
- Treble roll-off
- Intimate soundstage width

After Oratory1990 Harman Target EQ
Pros
- The detail retrieval shines and clear ( Before EQ, the details are hard to pick out, because of the warm/dark signature. But it's not missing anything)
- Very neutral sound signature
- Even better transparent and soundstage depth (Let's me explain the transparency, For example, where the instrumentals are overlapping each other, and I can clearly hear the separation between each instrument (example vocal and guitar are overlapping the middle, I can hear that the vocal is more forward than a guitar. While another track, a guitar is more forward than the vocal. So this phone can show how it is recorded or engineering.)

- Treble is very neutral, not roll-off, and not harsh.
- Bass and mid are very neutral, not too much or too less
- Still keep the bass slam
- Better soundstage width than before EQ, very neutral, but not as wide as HD800.
- Still keep the speed
- Still keep the imaging

Cons
- Losing the magic of female vocal (Before EQ, it's a bit shouty, but after EQ it's a more neutral sound, the graph also shows 1k shouty)
- Stax fart


Comparison
I currently own all of these three cans. A/B Testing with them directly
KOSS ESP 95X with Stock amp
This can is a mid-centric with mid shouty. I can't listen to it without EQ. After EQ to the Harman target and comparing with O2. It sounds harsher, with fewer details retrieval and speed. Very clearly not transparent as O2. It sounds more coherent and difficult to pick out each instrument. The bass slam is better than O2 but less bass quality (with Harman EQ) But when I boost the O2 bass with EQ, it's clearly better on O2. The only advantage is it is very lightweight(some this is cons), more comfortable than O2, and my ears can breathe better due to the seal.

HIFIMAN Ananda
I really love the Ananda tonality. The treble is one of the best I've heard to date. But after EQ, The O2 is just better over anything, except the Ananda has a taller soundstage which some may prefer. Also since it's planar, the sound has more body. Listening to some music genre (especially hard rock) will be more fun with Ananda.
The soundstage depth(or transparency as I mentioned above (1)) is not that good on Ananda comparing with ESP 95X and O2.

Audio Technica ATH-r70x
The sound has more body. The tonality before EQ is more neutral by my ears. Those are only two advantage, other it's just lose to O2 completely.

Conclusion: If you want the best of O2, Please unlock its potential by doing the EQ and you will be amazed.

If performance is based solely on ultimate transparency, then this is the best headphone I've ever owned and/or listened to. That includes the SR-009. Superb clarity and resolution without any hint of treble brightness or fatigue. Its purity of tone is simply astonishing; it's smooth, organic, airy and fast all at the same time. It adds little to nothing of its own, and neither does it subtract anything from the richness of the music. And for an electrostatic headphone, the bass performance is superb. Tight (!), fast and impactful with plenty of warmth and body. It extends all the way down into the lowest octave, albeit not with quite the same weight and force of an Audeze planar. 

The 007 is built to last, with materials being mostly magnesium and premium supple leather. Solid to the touch and highly comfortable for long listening sessions. 

Very demanding of amplification quality and power, especially in terms of getting the bass right. But rewarding in the end. 

"Farts" when moved/pressed on the head but it's a small price to pay for the sound quality on offer here.
 
I love this freakin' thing. You will NOT be disappointed with this headphone. 

 

This is not really a review, just some inital impresions I'd like to share with fellow head-fiers. I've spent some hours with the SR007 mkI and the SRM007 amp. I also compared the mkI and mkII briefly, and main difference, though subtle, was that the mkII is slightly more forward and bright sounding. I preferred the slightly more smooth mkI. 
 
I've listened to the Stax O2 a couple of times before, but recently I was lucky enough to take it home for trial. When I rigged up the stax rig next to my HD650/Woo WA2, I was eager to see how much more I'd like the Stax rig.... Here's my short comparison
1) o2 is of course substantially more detailed
2) o2 has blacker background
3) o2 has better soundstage, but not by the large margin I'd expect
4) o2 has deeper bass by far and makes music that I really only enjoy on large speakers shine
5) HD650 is way more comfortable. Main issue is velour pads and that the o2 cups don't swivel
6) HD650 is to me less fatiguing. Every time I switched back to the HD650, I felt relief.. The Stax  creates a slight vacuum, and is brighter feeling (though it isn't bright by any high end standard)
7) HD650 has better midrange tonality. The o2 has more detail, of course, but the HD650 has a better flavor. The o2 feels recessed, presenting a "neutral" or "cold" midrange, when the HD650 is warm and natural feeling. 
 
Despite the obvious technical advantages, I preferred the warmness and natural feeling of the HD650, though I really miss the smooth treble detail and powerful bass extension of the O2. 
 
Again please take this "review" with a grain of salt But I wrote it as a reminder that expensive and technically superior isn't necessarily subjectively better - cost no object...

Some love it, some don't but there is little doubt that this is one of the top-tier headphones out there and one that is not so rare, so if bought second-hand the price of the headphones and amp are very much comparable to a top-level dynamic/planar setup which makes these a compelling option.
 
The fit is a little tricky at first, but once it is set up it feels very comfortable on the head. It is important to position the transducers close to the ears. The build quality is pretty solid and the finish looks great apart form the cable entry which is prone to break if one is not careful.
 
Technically it carries all the aspects e'stats are famous for. So it is very airy and presents masses of detail but in a fairly unobtrusive way. It is quite sensitive to what components are in the signal chain and I would say, the simpler it is, the better.
 
The O2 Mk1 definitely has a distinctive sound that some may call "warm". That is, it injects life and emotion into the music even when it is not recorded that way and adds a bit of extra weight to the instruments. It has that creamy, smooth type of presentation which is easy on the ears so which makes it more forgiving, apart from a treble that is slightly uneven and can get "caught out" on some "harder, more digital" recordings.
The bass has a good amount of texture and weight and fairly punchy by electrostatic standard but could be more controlled.
 
Because of its own sound character and power need to control the bass, a linear powerful solid state amp should be a perfect match, however if some would like an even more "old-school" tubey sound they might like it with a SRM-007t or other hybrid/all-tube amps as well. The SRM-717 is somewhere in the middle carrying the solid-state refinement but also adding a little extra warmth to the O2 Mk1's generally warm sound and makes the soundstage slightly diffused and it does not have full control over the bass.
 
 
So how could I sum this one up?
Well, I think the SR-007 Mk1 is definitely "The Dark Knight of electrostats"
Just listen to the main theme with these, and you will hear what I meant So if you happen to like that sort of thing, you will not be disappointed.
 
(Main test setup was: Lavry DA11->SRM-717->SR-007)

The first thing I noticed with these headphones is clarity. -These are the only headphones that I have heard that can execute perfect imaging while generating a fully independent bass sound right in the middle of the soundstage.
This is the first time I have heard sound so holistically reproduced, and as an audio engineer I find this to be especially inspiring.
With other headphones, (particularly dynamics) the bass distorts the coherency of the sonic image and two sounds often compete for the same area of soundstage.
Comparatively, the 007 have uninhibited resolution and inner layering of detail which coalesces into a holographic audio image.
It's hard to imagine headphones being any better, and when it comes to amplification this is just the tip of the iceberg for me and my 717.
The best amps available are the Woo WES, Headamp KGSS & BHSE and the DIY KGSSHV.
Although the 717 is the budget amp for these headphones, this rig has changed the way I listen to music, and how I form criteria for gear I audition.
 
Before arriving at electrostats I built a balanced dynamic headphone system, owned the AKG K340 (hybrid of dynamic & electrostat), and the YH100 (orthodynamic).
So far the electrostatic technology that I have heard is the most advanced, although I am also watching out for new orthodynamic offerings as they become available.
At this point I have largely lost interest in dynamic headphones.

Headphones simply do not get better than this.  The first time I heard the O2 mk1 was at the First National Meet.  I was blown away.  I had no idea that headphones could sound like...that.  They simply melted away.  It some ways, they ruined that meet for me because they were among the first phones I heard.
 
Fast forward a few years and a few meets later and I decided that I had committed to enough money to mediocrity, and I finally pulled the trigger on a pair on the used market.  Amped with a KGSS DX, they are a splendid mixture of power and finesse.  "Liquidy" is a word I associate with them.  And there is not a single thing I would change about the sound signature.  They are, simply put, the best headphones with which I have spent a meaningful amount of time.
 
The O2 mk1 is demanding, though.  You will need to invest in a 'stat amp of some quality and feed it a decent source.  But that investment will yield sonic bliss.