There's a great article about this in the current Absolute Sound and the limitations of USB audio. But the BelCanto USB interface, that then is meant to go from the USB interface into a DAC, improves the sound tremendously. I don't have one, since I don't use USB as an output, but a guy in our audio society did a test using it and there was a huge difference. He used the BelCanto USB interface from a MacMini into a BelCanto DACIII and the differece with and without it was incredible.
