I dunno. I didn't use the 320hm 990, but the 32ohm 880. I had the 600ohm DT990, which played fine off the Mixamp alone if game/voice is set to game 100% (meaning no voice chat). Fine as in most of the time. It did tend to clip/distort during some bass heavy scenes, but the E9 between the Mixamp and the cans helped out with that.
Eric_C, I never used the MMX300, let alone the 32ohm DT770s, so I don't know how well they perform. Can't recommend something I haven't tried myself. I only used the DT770/600 and DT770 Pro 80...
I can say now that the 600ohm 990 does indeed sound worse than the 32ohm w/ the mixamp. It was alright for shooters, had the "theater" presentation you were talking about, but with some other games like Brutal Legend, Oblivion, etc it does not sound right at all. The 32ohm I already did not like much but that was still better. Strangely enough, the T1 is also 600ohm and sounds so good through the mixamp, and the 600ohm dt990 does indeed have less recessed mids/better treble with music, but with games the mids are more recessed than ever.
Is there any way to hook up my HDP or E9 to a 5.8 mixamp? If not, how do you hook it up to the wired mixamp?