Pros: Top-notch build quality and design. Supremely comfortable. Accurate and utterly effortless sound.
Cons: They are extremely expensive. They're still headphones.
First of all they are drop-dead gorgeous and an extremely high quality construction. I have not seen any headphone close to this type of quality feel. It can't be described. When you hold these you won't want to let them go. They are a stunning masterpiece in this regard. They make every other headphone feel cheap. Just... wow. Top that with superb comfort and you've got a an object that will send small pulses of pure joy through your body every time to come in contact with it.
The sound of these headphones didn't wow me the way I was expecting. I've come to the conclusion that no headphone is physically capable of delivering music with quite the level of realism a great pair of loudspeakers can. After all the very positive reviews I was expecting a sound that would knock me off the chair. Well, that didn't happen. But these are still a fabulous pair of high-end headphone goodness.
Having heard a lot of high end headphones and several STAX models I can say the SR-009 sounds more like a perfected version of the old Lambda sound signature than a shot for a warmer, more honey-rich sound like the SR-007.
The first thing that struck me was the level of precision with which these played the bass notes. I don't think I've heard a headphone with bass that sounds this distinct and precise before. Über-tight, lightning fast, hard-hitting, very clear and with excellent extension, these have the best bass reproduction I've heard in headphones alongside the LCD-3, which provides (even) better low-end extension and a more visceral, realistic heft. It's a matter of taste, I would say.
The top-notch clarity and precision continue to the midrange and treble, during my short listening sessions I found no hint of even the slightest grain or imperfections. These are, if anything, extremely even sounding. No peaks or dips, nothing stands out. Everything just comes together.
I could write more on the different aspects of the sound, and the sound-stage and imaging (which are both top-class as well) but I really see no point in it. These excel in every way, and their pride is not in how they do everything so extremely well, but in how they do so little wrong.
If you're looking for the most technically perfect headphone available, this is the one without question. A friend of mine prefers the sound of the SR-007, stating the warmer, more chesty sound on offer there is the more true to life. Personally I find the midrange of the SR-009 more distinct and up-front, with greater depth of field that gave a better sense of the space and air to vocals and instruments. It sounded more "real" to me.
So far, I've only heard one case where audio equipment managed to reproduce a perfect human voice. That honor goes to MBL's X-treme loudspeakers connected to 2 pairs of their reference power amplifiers and their reference CD-player and pre-amp. That system is almost $200 000, if I'm not mistaken. The SR-009 doesn't come anywhere near that realism, and that is their biggest flaw. They are still headphones, and considering their cost which can grow to astronomical numbers depending on what source/amp you plan on buying for them, that may be something to be aware of.