Personally, I did not find the AD700X lacking when playing music. It has a bright and airy tone, which translates to clarity in for example, string instruments (both acoustic and electronic) and perceived large soundstage. I think the large drivers which are also angled may also add to this effect. (fyi I've some photos in my profile of the inside of the AD700X)
I listened to and enjoyed the AD700X with all genres of music, and for its price range I generally found them excellent. The mids and bass was quite well defined, but I agree that AT phones are not neutral sounding, and many people do prefer more bass.
In a gaming environment, the thinner tone certainly was helpful in detecting sounds, in particular footsteps and movement of enemies nearby. You would think mid and bass tones would be more important for footsteps, but personally I found the treble sound more easier to detect - footsteps have more definition, as well as the rustling of clothes and vegetation etc. (think of battlefield 3 and 4)
What I would say is that the headband design on the AD700X may not be comfortable for everyone. I found the AD700X did not fit me very well, and the earcups ended up sliding down my head and resting on the ears. The headphones are very light, but it was still uncomfortable and annoying. The AD900X may share the same headband design, though I'm not 100% certain.
The cable also never fully straightened out which can also get irritating.
Good luck with your purchase!
Edited by SunTanScanMan - 5/17/14 at 7:54pm