Hey, I'm probably one of the few that can chime in on this as I've owned both the SR-80i and the AD900(close enough

) and used both for gaming.
The Grados I found amazing for gaming actually, they sound a little off in games, but if your playing competitively your be very happy with your purchase. I was playing Crysis multiplayer and was able to detect an enemy all the way across the map(a small map) by the faint buzzing sound of his invisibility cloak. He was crouched behind a crate up against the side of the map, I headshotted him the second I came around the corner. I believe the game went 10 to 0 before he gave up, Grados are good for this kind of gaming. Grados are not bass monsters, but they do great with the 'punchy' bass gunshots and explosions need to shine. The rumble of your ship in ME will be lacking in feel but will still sound great. Headphones with great bass will put you next to the ship.
Now for the AD900s, they sound big, as I was walking through a cave in minecraft I could hear the sound of my footsteps echoing throughout the cave, very impressive. They too have pretty good accuracy within the soundstage, but I feel that the Grados did better for competitive gaming. The 'airy' sound of the ADs was a bit distracting and the lack of bass made gunshots and explosions sound flat and distorted. Back to the ME example, the rumble of the ship sounds fake and artificial.
For music I found the AD to have an addictive airy sound, but the music itself all seemed to clump together into a loud mess. The lack of bass makes everything sound off. I couldn't listen for more than an hour before throwing them off. Grado on the other hand, you will feel that you are in a small room with the band, nothing really will sound off and well different instruments will overlap a bit they won't be sitting right on top of one another like the ADs.
And in case your wondering, the AKG K81s that I have, while I find them to better the Grados for music purposes are horrible gaming headphones. For example, I'm charging an enemy in mount and blade, I hear footsteps behind me and spin around. I get a sword in my back before I realize that what I was hearing were my own footsteps poorly positioned a few feet behind me, not good gamers at all.
Edited by MohawkUS - 2/5/12 at 6:34pm