Dolby Headphone has 3 room modes. Most devices use Room 2.
While testing Nameless's Victor SU-DH1 (which has all three DH room modes), Room 1 was the smallest virtual space, with the least processing. Room 2 was the typical DH mode used in most devices like the Mixamp. Room 3 was an echoey mess, with a metric ton of reverb.
While I can't compare yet (I might win that Mixamp tomorrow), this DH reminds me of Room 1. Highest fidelity for DH, but not as airy as Room 2, which hurts positional cues. AGain, I'm just assuming. I did just fine with the receiver yesterday on MW2, and Avatar sounded AMAZING with it. It may as well be room 2. It's been over a week since I sold my Mixamp 5.8, and my memory is hazy.
Ahhh, I see now. Maybe that is why the Monoprice amp sounded so bad in comparison to the Mixamp. Room 3 pretty much describes how their amp sounded.
Back to the PS3 issue, have you tried this yet? Have you tried a different hdmi cable as well? My roommates GT30 wouldn't even work with the PS3 directly until we switched to a different cable when he first bought his TV.