Pros: Super comfortable, smooth treble
Cons: Veiled midrange, weak bass, limited availability
70mm drivers, fully open design, premium materials, and simply the best comfort of any headphone at any price. The MA900 has many things going for it. The sound signature is a little laid back, but ever polite. One would think that a 70mm driver could provide a hearty slam. This isn't the case -- sub-bass is almost nonexistent, and midbass is inferior to that of a Grado. Decay is slow on the MA900, so the midbass isn't super-crisp. Treble, on the other hand, is quite smooth on this headphone. It's not a bright headphone, so even the shrillest of instruments, such the piccolo, won't pierce your ears.
The big problem, however, is the midrange. It's clouded. At $350 (imported from USA to Canada in June 2012 -- Sony Store only carries up to MA500) I wanted a headphone that was crystal-clear. Unfortunately, I think the thick fabric covering the driver has ended up muffling the headphone a little. Soundstage width is a little congested unless the MA900 is worn in reverse. But in reverse, bass extension and impact are reduced even further.
I suspect the frequency response is tuned to more of a studio monitor headphone than a musical headphone. Unfortunately, it means the MA900 is miserable-sounding at a low listening volume, as the bass is too neutral to provide any spark to the music. Ultimately, while the MA900 is a good headphone if you blast your music, I like to normally listen at <75dB SPL. For that, my Grado vented SR225i (2 large holes, 8 small) bettered it in most aspects, as its frequency response better follows the equal-loudness curves. I ended up returning my MA900.
There's a good reason why the street price has dropped from $300 to $200, and that Sony Canada only carries the lesser models (MA100/300/500) in stock. It just isn't very good. The lesser models are even worse, however, so I don't know what Sony's thinking.