Why USB to SPDIF and not optical?

FYI, y'all:
My system - PC, Windows XP, ASIO4ALL, JRiver Jukebox > Belkin USB out to Giga Lab Moon USB DAC(NOS). My DAC also has coax and optical ins.

On a whim this afternoon, I inserted a $30 Behringer USB headphone amp (which also converts USB to optical) between the PC and the DAC (optical from the Behringer to the DAC). I was amazed to hear at least a 10% sound improvement, especially on my FLAC downloads from HD Tracks.

Apparently, my DAC prefers optical over USB, but I don't know why.

Hey dtc.

"The purist in me would still like to get the original 16/44.1 out to my DAC without the any changes happening in the PC."

Why do you prefer 16/44.1 over the higher resolutions? I am under the impression that 16 bits is better than 24 and 48kHz is better that 44.1 and 96 is better than 48, etc...
Am I missing something?

Hey Steve.

Why is Kernel Streaming better than ASIO for KMixer avoidance?