You are already using 6 channels

front left, front right, center, rear left, rear right, sub. Even if the GTXP has support for 6.1 (added rear center channel...I recall it being able to do this, although I'm not positive...I haven't followed the GTXP closely), unless you had a lot of money to burn, adding a rear center really dosen't have much of an effect on the sound.