The Mixamp is a gaming amp. The O2 isn't. They're nothing alike. If you want to plain in plain stereo, you go for music amps. If you want the additional benefits of Virtual surround for gaming, you get something like the Mixamp pro.
There is no comparison. if you want virtual surround, get the Mixamp. if you want just absolute stereo fidelity, then get somnething like the O2. You lose the benefit of virtual surround, just to get better sound off stereo. I personally prefer virtual surround than losing that extra dimensionality for better stereo fidelity. Not having virtual surround is like playing with blinders on in comparison.
If you want to additionally drive your AKGs with more power for the Mixamp, then attach the O2 TO the Mixamp Pro. That isn't necessary for gaming unless you're hooking up a mic with your headphones, because the Mixamp has to split it's power between driving the headphone, and driving the mic channel.
When gaming, absolute fidelity isn't as important because you're splitting your focus between sound and whatever is on screen. I can game just fine with the Mixamp alone and the AKGs, when I'm not using a mic.
I've mentioned this a million times before. Seems to be asked every other week.Compare DH off, to DH on at 1:08.
If you feel it's just fine with DH off than when DH is on at 1:08, then just get the O2. If you liked what you heard in 1:08, then get the Mixamp pro FIRST. End of story.