Nucleus can output directly to the DAC via USB. In general, USB is not the best connection between a music server and a DAC, because it is difficult to keep server electrical noise from traveling down the USB connection to the DAC and contaminating the analog side of the DAC. Some combinations of server and DAC work fairly well (for example, I got surprisingly good results with a Raspberry Pi 2 feeding a Soekris dac1541), but other things being equal I’d rather use S/PDIF from the source to the DAC than USB for that reason.