I have a pair of Astro A40's which is the same as the A50s just without wireless and I can say that their highest redeemable quality is their comfort level, Like pillows on my ears and I can wear them for hours without getting uncomfortable. However from what I have heard, the STX+DT990 will provide far superior sound quality in every application. If you are considering the A50's for gaming, don't. The so called "surround sound" is really just garbage. I have used a whole plethora of "gaming surround" headphones and nothing beats a good pair of stereo headphones.
If the built in mic is attractive to you, try getting a decent clip-on mic and attach it to the DT990's cable. With all this said, the A50's will still perform well for gaming, movies, and music. I've had my A40's for two years now and I've worn them every day. (no joke) I've had the opportunity to use them for everything and they are just well rounded. And if I were you I'd just get the A40's without the stupid mixamp. You can get a pair of them for like $150 without the mixamp I believe. I feel it necessary to mention the bass levels are naturally low with the A40s and A50s unless you have the volume turned up but then again that really comes with all "gaming" headphones.