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.