If money is no object and you want the best, I'd just bring up Stax, but that's arguably overkill for gaming, even the vintage $250-300 Lambda setups.
With so many people decrying the AD700 these days (they can't stand its bass deficiency and use EQ to get a similar sound signature for competitive gaming), I'm not even sure what to suggest when increasingly popular headphones such as the SR850 and aforementioned FA-011 are ones I still have yet to try.
I don't know if the Mixamp acts as a PC DAC/DSP through USB, but if your motherboard's audio codec supports Dolby Digital Live, you can use that to feed the Mixamp. If not, you could get a Xonar DG on the cheap end, or if you play some older PC games that make use of EAX, go seek out an X-Fi Prelude, Forte, or Titanium HD.