So I've been running an optical output from a USB sound card (sound blaster
MP3+) to a Zapco DAC for a while now with great results. Today I tried to
use the coax spdif output of the on-board sound card (Via ITX board - AC97
codec) with the latest drivers into the same DAC. Oddly enough, it sounded
awful. There was a prominent high frequency distortion generated by the
midrange and highs of the music. It didn't sound like clipping distortion
or anything like that. It sounded more "digital" in nature - almost like
high frequency artifacts introduced by a poorly encoded mp3, only more
severe.

How can this be? Is this originating in the onboard sound or the DAC? I'll
be installing another DAC soon (Behringer), but it might be helpful to know
beforehand whether or not I have to keep the existing Sound Blaster USB
device installed.