kdl, your build idea is right on point. The use of the USB is a good option, but personally I'd use a dedicated DAC using the optical output of the MB. This will reduce noise trememdously (not talking ground loops, but noises inside the PC like HDD wind up, etc.). I use this setup in my system. Other than that, you appear to have a well balanced design with enough power for all speakers. Seems like a good build to me.