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.

Mmarvin19 - The JRiver and ASIO configuration is one I am intertested in but have not tried yet. Can you tell if you get 44.1 MHz out of both the Belkin and the Behringer? And do both pass higher frequency without re-sampling? I check this by hooking it up to my HT receiver which reports the input signal sampling frequency. Thanks.

"XP generally upscales 44.1 files to 48 unless special drives are installed."

Not true. My USB converter uses the native drivers and it outputs 44.1 on XP. There are several ways to avoid kmixer, including:

1) unmapping the device
2) Kernel Streaming
3) ASIO

Steve N.
Empirical Audio

Steve - Let me re-phrase - "XP generally upscales 44.1 files to 48 unless special software is installed to bypasses the XP upscaling". You are correct that is not only a driver issue. Unfortunately the simple solution of unmapping the device often disables the device so it is not a general solution - at least for me The specific issue I was addressing is whether the Bel Canto managed to bypass this upconversion. If there is no additional software other than the native drives, I would suspect that the upconversion still happens. Do you concur? Does your converter get 44.1 out of iTunes? As I understand it, neither KS or ASIO is available for iTunes. If that is incorrect I would love to know a general process for bypassing the upconversion when using iTunes. I like the iTunes interface, but am frustrated by the uponversion. Any insight would be appreciated.

Correction - not mapping through the device does not disable my devices, it just does not seem to stop the upsampling. The "do not use audio functions" option is the one that disables the devices. Sorry - I got the 2 options mixed up.