Damn, after doing some more research on their forums I found out to be able to get 5.1 with the mixamp, you need a soundcard that puts out DDL which astro fails to mention on their website. I might have to keep looking for another headset .
Well to experience "true" 5.1 out of any system you need a system that does Dolby Digital or DTS. The Mixamp does emulate 5.1 surround though with the Dolby Headphone technology which I've found to be very good. Even if you get a sound card that supports 5.1 though you'll also need a source that has 5.1 - MP3's won't, most TV shows/movies won't unless ripped from Bluray.