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.This has been my experience also. I think just about anyone who has used a usb to spdif converter wouild agree that it is better (in almost every case)than using built in Asynchronous usb on the DAC.
