You'd be using the optical output of the onboard audio codec, at the very least, but obviously none of the analog outputs. I still don't know if you can actually send a real-time-encoded Dolby Digital signal that way, though (which is what Dolby Digital Live does). On the other hand, the Mixamp has a USB port IIRC, so maybe you could use it like a USB DAC.
Does that someone you asked own BOTH the Mixamp and the X-Fi Forte? Ask him to set up the X-Fi Forte to output Dolby Digital Live through S/PDIF (it has a Toslink adapter for the RCA jack bundled with it) and into the Mixamp. There's just too many variables here, and I want to find out if it's more of a preference for Dolby Headphone over CMSS-3D Headphone in that person's case.
Well that person sold the x-fi forte for the mixamp, now he has the mixamp he says he prefers it. Although he did say it was mostly because he liked Dolby Digital better than CMSS-3D!
But theres no point in me loosing out money wise to something that will be very similar in SQ! Also i heard that if you have onboard sound it could be quite noisy compared with external DAC's and amps!
The motherboard i will be getting has Coaxial SPDIF port and a Optical SPDIF port, also says it supports 7.1 channels surround sound playback