That's what i'm doing...
I have the gametheater XP and i just ordered the AT 5.1. The main problem with the GTXP is that there are 1/8 miniplug outputs for front and surround channels, but only RCA outputs for the center and subwoofer. I'm sure you could get an adapter somewhere to solve this problem...but it makes it pretty complex...
I guess if you could find a 2 RCA to miniplug input adapter to plug the 5.1's center/sub miniplug and then just use the regular 1/8 outputs for front and surround? I don't know. There's evena a chance that you would need to buy adapters for the front and surround RCA channels also if the miniplug outs don't work at the same time as the RCA outs...
I figured the easiest way would be to get the AT 5.1. This way i can just use the digital toslink cable from the GTXP. This still requires one adapter, the one that klipsch suggests in another post.
Advantages here are:
Remote control for volume...(cause the XP does not have a remote like the SB live platinum)
Delay settings for the different channels...(my rear speakers and my couch are against the back wall of the room i'm going to set this up in, so i'll need to delay the rear channels). You can't delay the channels any other way than getting the AT 5.1.
All in all, if you have the money, i think the AT 5.1 is a good thing to buy. Much simpler setup, plus it digitally detects the AC-3 signal so you're ABSOLUTELY SURE that you're getting AC-3.