Koestner - some USB interfaces are constantly improving, even my own. I have done at least 2 mods to the Off-Ramp and added an optional power supply in the last year. It is 5th generation, Off-Ramp 5.
The thing to realize is that an external USB converter like an Off-Ramp 5 driving your USB DAC with S/PDIF will undoubtedly beat your USB interface and probably your transport too. I have lots of customers using the Off-Ramp 5 to drive USB DACs, abandoning the built-in USB interface. 30-day money-back, less shipping.
Off-Ramp 5:
http://www.empiricalaudio.com/products/off-ramp-converter
Steve N.
Empirical Audio