Comparison review is up: http://www.head-fi.org/t/650499/comparison-review-ath-a9x-ath-a900x-akg-k550-three-way-shootout
Will burn both cans more and update if I detect sound quality changes. I think the A900X will be a better choice for gaming both due to louder bass (gives gunshots and explosions more impact) and I personally felt sound positioning is more focused/precise on the A900X. K550 has a very large, airy sound stage that envelopes you but sound image is also a bit blurrier.
Damn, deciding actually got a little more complicated with that review!