If you are using XP, I would recommend unmapping the device. This will make the USB interface sound a bit better. USB interface quality varies a lot from one product to another, so it may stil not be better than the S/PDIF input. Here is how to unmap:
Go to Control Panel - System Hardware - Device Manager - Sound Video and Game Controllers Empirical 1.1 - Right-Click and select Properties - Audio Devices Empirical 1.1 - Properties - Set "Do not use audio features" and set "Do not map through this Device"
You will need to reboot and then if you change it back to mapped, then you must re-boot.
Steve N.
Empirical Audio
Go to Control Panel - System Hardware - Device Manager - Sound Video and Game Controllers Empirical 1.1 - Right-Click and select Properties - Audio Devices Empirical 1.1 - Properties - Set "Do not use audio features" and set "Do not map through this Device"
You will need to reboot and then if you change it back to mapped, then you must re-boot.
Steve N.
Empirical Audio