DH1, 2, 3 (room modes) are different settings for Dolby Headphone. They are NOT on the Mixamp. The Mixamp uses DH2, as that's the 'best' mode. DH1 has less of a surround effect and the 'room' where the sound is spread around you is smallest. DH2 is what all console DH amps use, and I've been told that it's the best of the three. DH3 sounds like you're in a big room, with a lot of echo/revervb, and sounds more processed.
If you're using the headphone setting (IN GAME options) in black ops, you're not getting Dolby Headphone at all. Those types of options are either turned back to 2 channel audio optimized for headphones, or adds it's own software surround emulation which doesn't come close to Dolby Headphone, but if you think its better, sell your Mixamp and stick to that. That is, unless you were using that headphone mode WITH the Mixamp... in which then you were using two different surround processings at once, which SHOULD sound like crap. But hey... I'm not you.
Black Ops has it's best DH audio when you set it to Quiet + Theatre. That is the general consensus, and one I agree with.
Oh no **** eh! So turn on DH on my mix amp. Change the xbox to 5.1. Then black ops to quiet + theater? Is this correct?