You mean like for music? I can't test it as my computer doesn't have spdif/optical out. You can always get the FiiO D3 for $30. It takes coax and optical in and goes to a line out. With the mixamp, it doesn't have a line out, so you're stuck with the amp/headphone out for whatever the DAC gives you.
but the question remains: does the D3 or any other comparably cheap DAC have noticeably better SQ than the mixamp. Would I really benefit from the d3+amp combo compared to the double amped mixamp? with 320kb mp3s that is.