The HD598 does NOT need to be amped past the Mixamp for gaming. It however should be amped for MUSIC needs (not with the Mixamp attached), which I assume he listens to music away from his future Mixamp setup. I'd advise on NOT using the Mixamp for anything related to music.
The A40s don't compare to the PC360, IMHO. I'm not alone in saying this. The A40s are overrated and overpriced.