I found the AD900 a terrible headphone in every way, it has no bass...the soundstage sounded congested, it sounded like all the sounds were cramped together when there was a lot going on, what was great about them though is the sense of air behind the sound, but air won't help you in competitive gaming, you want accuracy.
"A terrible headphone in every way" is a bit hyperbolic, don't you think? Not your cup of tea, maybe, but that's an entirely different thing.
To give a dissenting viewpoint - I find the AD900s extremely good. Do they have a ton of bass? No, but then in my opinion that's not what you want when trying to discern detail. Remember that he is talking about competitive gaming, not single-player gaming, which are very different beasts. That said, they still have enough bass for me (which the AD700s do not, incidentally).
I would also dispute that they have a congested soundstage, I don't find that at all. But then, you're comparing amped headphones (PRO 2900s, 598s) with an unamped headphone (AD900) and of course amping is going to improve things, including soundstage. Apples and oranges.
But he wants to use them unamped, which to my mind says the AD900s.