If you already have a USB DAC that also has a S/PDIF input, you can usually improve the performance by using a good outboard USB to S/PDIF converter. Most USB DACs have poor implementations of USB interfaces. A good 1.5m long S/PDIF cable is required. The better the cable, the better the result. Also, the better the power supply for the USB converter, the better the result.
Steve N.
Empirical Audio