Pros: World-class build quality. Extremely comfortable. Detailed and utterly effortless sound.
Cons: Ridiculously expensive.
STAX have always been the choice when it comes to ultimate transparency through headphones, thanks to their superior electrostatic designs. When you listen to a good STAX headphone, suddenly everything sounds more like live music. The soundstage is perfectly defined, dynamics are explosive yet effortless, and the tiniest nuances and textures in the music come through clearly as can be with an absolute naturalness - all with a smoothness of sound that makes even the best dynamic headphones sound somehow rough around the edges.
In some ways, the SR-009 is the best headphone STAX has ever produced, but not in every way.
Starting with the build quality, its definitely a big step up from earlier models, including the SR-007. Materials and assembly exude a remarkable level of precision, with solid aluminum and soft leather of the highest quality. The earpieces are held together by the arc in a less "fixed" fashion than most headphones, which results in the SR-009 kind of hanging on the sides of your head. This makes for a loose fit, but also excellent comfort. Definitely a better fit than the SR-007, in my opinion.
The sound signature of the SR-009 leans more in the direction of the older Lambda series than to that of the SR-007, but with a somewhat firmer bass foundation than the Lambdas. Bass texture and definition is spectacular, and low-end impact is actually perfectly satisfactory. It just feels like the drivers are extremely well controlled, because the bass just seems so agile. Extension is also first-rate. The bass, while punchy, is lighter in mass than that of the SR-007 mk1, which gives equally airy but more substantial and in my opinion more realistic bass.
To give some perspective on how much bass quantity we're talking about, expect something in between the Sennheiser HD 800 and Beyerdynamic T1.
Midrange is right where it should be in the mix, with a wonderful acuity and airiness to vocals and instruments. I do feel, however, that there is something missing. There is a slight absence of warmth in the sound, leaving vocals in particular ever so slightly lacking in organic presence. It's a slight deviation, but at this price one can't be anything but picky with such details. If you find the midrange of the HD 800 to be tonally spot-on, you'll appreciate the SR-009 too. If, however, you think the midrange of the Audeze LCD-2 rev 2, SR-007 and also some Lambdas is perfectly natural, I think the SR-009 will sound just a tad sterile to you. And I do not find the clarity to be beyond what the SR-007 can accomplish when its driven to its full potential, as some people claim. But that's just me.
Then there is the treble, which I think is excellent. Fast, airy, detailed and everything one can expect from a great electrostatic transducer. While some will argue about it being brighter than optimal, it's not forward relative to the midrange and bass and feels as even and extended as one can even wish for, without ever sounding sharp or strained. To my ears it's right at the spot to provide accurate imaging without overdoing high-frequency instruments. Bright or not bright, the SR-009 certainly has more sparkle and energy to its sound than the SR-007. While less warm and inviting, the SR-009 is the more lively sounding of the two.
Oh and speaking of imaging, its up there with the best too. Depth of field is exemplary, almost 3D-like (kind of similar to what I heard with the Beyerdynamic T1) with very good layering. The headphone distinctly portrays all of the individual sound components in the recording with no confusion whatsoever. While the cheaper STAX models never hint at confusion during complex music pieces either, I think the SR-009 is the best of the bunch in terms of pin-point imaging. However, the shape of the soundstage itself is more spherical and less stage-like than that of the SR-007. When you listen to the SR-007 well amped, it gives a very special sense of distance and acoustic timbre, like sitting in an actual room (for a headphone, that is). The SR-009 is more enveloping, and more forward. Less room-feel and more "space" feel. Some will prefer it, and some will prefer the SR-007.
As good as this headphone is - and it's excellent, no questioning it - I feel that, judging its performance relative to other STAX headphones, it doesn't justify the price. Die-hard audiophiles with hefty wallets should consider the SR-009, but if you're on a limited budget I wouldn't say the 009 is worth it. And (IMHO) a well-amped SR-007 mk1 may even be preferred. The SR-007 gives a warmer, more stage-like and somewhat more realistic presentation of music - whereas the SR-009 has a more monitor-like portrayal, a more spherical soundstage that puts music around you rather than pushing it up front and gives up some organic warmth in favor of a slightly more analytical (though not HD 800-analytical, or even SR-507 analytical) tone. I think the SR-007 does better in the midrange and in coming closer to what live music sounds like, but I think the SR-009 has more "oomph" and dynamic potency to its sound, mainly because of what I find is a more "snappy" bass and more treble presence.
In short, I would say the SR-009 is a technically perfected but slightly less warm sounding headphone than the old Lambdas (I'm talking Lambda Pro, Nova signature etc), and should not be seen as better version of the SR-007. It's not, the two headphones are quite a bit different and you'll just have to decide what you want and what you value most. The SR-009 is a much fancier construction, more comfortable to my ears and its sound will appeal more to some than that of the 007. The SR-007 requires heavy amping (the STAX amps in standard setting are not powerful enough) while the SR-009 will work well with the SRM-007TII. But if I had to choose a headphone to enjoy music in the best possible way, my personal choice would be the SR-007.