That's doable but personally it's easier if you use SPDIF from the motherboard or soundcard utilizing its DSP into the teh DAC-HPamp and then jut wire the mod mic into the mobo's or soundcard's mic input. If you do it this way then all you have to do is open Creative's suite and disable SBX Pro Studio, or revert a Xonar software suite to the Hi-Fi default, which automatically shuts off all processing.
Having the music 2ch path on USB requires going through Control Panel to select the output device. Not that much longer to do but while rare it's not impossible for those two audio output devices to have some kind of error or conflict, more so if the USB DAC uses anything other than the generic drivers. It's not that common but combined with how much faster you can switch this is an easier way to do it.
