I've now extensively messed with the Soundmax card that comes with the X48, and I can conclude that it's crap.
Muddled sound, dead highs, and for some reason they insist on putting front speaker sound through the center channel, and it overpowers the other speakers.
Unless you fiddle with their advanced settings, the 5.1 doesn't use the rear speakers.
Also, if the bass is turned up, it overpowers the other sounds to the point the sound fluctuates like someone is turning the volume up and down.
I experienced the same issues with my m9700 with the Soundmax built into the mobo, but I thought it would be different with a top end desktop mobo...
I'm going to try my old Audigy 2 until someone comes out with an affordable X-Fi based solution, that isn't a creative part.