Pros: Can go loud with harsh recording with little or no penalty, Grain-free; Very convinciong sense of space for out-of-head imaging
Cons: Some forwardness on lower mids lowers transperancy; Too polite treble
The MDR-MA900's the current the top model in the latest open-back line-up from Sony. While the latest generaraion is definitely not the greatest with a distinctly less "hi-tec" vibe then the previous SAx000 models (as perhaps can inferred from the one-digit-less suffix and the of course a much lower MSRP pricing) the MA900s, in particular as a TOTL model, comes with an altogether different concept- that of the "Full-Open-Air" design, Originally pioneered in the '90's with the MDR-F1 (by one of the principle desingers responsible for the legendary MDR-R10 by what i read in a certain blog, RESPECT).
How this design is differentiated from a standard Open-Back is the minimization of the chassis frame to just the base area needed to support the driver assembly and for the mounting of the earpads. The rest is left for the open-air to fill in- Think open-baffle speakers on the head… This serves for both sound-quality and comfort pourposes: Main objective is to minimize resonances both from interaction of intial sound waves coming from the driver and reflected waves form the head with the frame, thus achieving a less "boxy" sound-field. Second is to reduce total weight of the frame, thus achieving a better long-wearing experience. Major disadvantage with this approach is the weakend bass power because bass is omni-directional and with open-back headphones some reflection off the frame is beneficial for an adequate response (with potential added support from the earpads material). With the MDR-F1 Sony implemented a simple low-pass passive acoustic filter foam-like funnel to enhance the bass impact (you can read about it here) and while it contributed to a rather acceptable mid-bass response the lower bass was still mostly MIA. For this reason probabley, the MA900 has gained a 40% increase in diaphragm size to a (presently) whopping category-leading 70mm (2 4/5"), With next largest AFAIK HD800's 56mm. This of course does not imply that size alone matters for quality and while MA900's bass is indeed a major improvement from the F1 it still is lacking in authority in the lowest octaves (20-60hz) compared to some other open-backs. It is mid-bassy rather than plain-bassy, but in the context of it's design goal as described above it's is very good and I'd never describe it as an anemic response. To relate some sense of reference I'd place it as noticeably above the border-line anemic ATH-AD900 and slightly ahead of MDR-SA5000 in the upper bass presense.
What defines the character of MA900's sound is of course how open it sounds- A very strong sense of sound that is originating outside of the head and in front of it, almost as if projected from a miniature soundbar on the forehead rather than coming from stereo elements hanging just next to the ear. It's not as vast or extremely spacious (HD800, K701) but in terms of a sheer-openness scale it's got to be one of the best I've heard from a conventional headphone with earpads , barring electrostats. Whether there's an added psycho-acoustic side-effect from how light and non-intrusive it sits on the head I can't tell for sure but it the it's definitely something more convincing even compared to some very strong open-backs as the AD900. I feel that the mild treble response holds aback this "disappearing trick" some, a more neutral (=brighter) treble response would've done nicely to futher illuminate the notes' overtones and improve the imaging accuracy which is only slightly better than average, About on par with the F1 and AD900, but clearly inferior to a laser-etch grade projection of the likes of SA5000, ATH-AD2000. On the upside it depicts a very cohesive soundscape with absolutely stellar center-focus.
MA900's tonal quality is a mixed bag for me. The moderately forward lower mids, along with the added bloom reminiscent of the mid-bass hump and the very polite treble response do hinder somewhat the sense clarity and refinement of an otherwise a remarkably clean and grain-free sound. I do wish Sony would have continued the reference-type sound sig from the SA5000 in the frame of of Full-Open-Air but I realize that it was not the intention from the very concept's beginning in terms of Ergonomics and technical restraints such a frame puts on overall fidelity. Perhaps if this concept would hopefully be continued in the future that a more advanced diaphragm material could be utilized instead of the "old-tech" Mylar PET, such as the LCP (Liquid Crystal Polimer) from recent models such as MDR-1R, MDR-Z1000 and EX1000.
Regarding the build quality: Many describe these as flimsy and cheap feeling. I partially agree. The frame indded doesn't retain a rigid composure such as similar models in the price range may, Certainly MDR-F1 and ATH-AD900 IME. A notable "flaw" shows if the cans are held single-handedly from one cup the other will "flop" sideways. However, this design has to be taken in the context of Sony designers trying achieve the lightest frame and a minimalistic form-factor at the same time. So having definitely tackled both these goals with great success I personally thin the flimsiness serves it's purpose and it's totally forgotten as soon as they gently conform on the head . Another common rant, as plasticky as they seem, the most vulnerable/thin areas which are the arms and forks between headband and cups are made of magnesium, not plastic. My only gripe would be the stiching of the headband could be better and I saw a couple of loose ends just a few weeks since I opened the box.
To conclude my impressions, I think the main selling point of these headphones lies not in sheer technical capabilities but in combining an incredibly light and comfortable full-sized from factor with a nicely articulate yet smooth signature , and especially with a great staging ability to make the music float (almost) as convincingly as if it was originating much further away than few cm from the ears that the enormous 70mm transducers are reproducing it. These traits do add-up for a superb all-purpose headphones at a very competent price.