In asynchronous USB the transport is slaved to the clock of the DAC. The clock is all-important as good clocking is the only viable antidote for jitter. Nevertheless the quality of the USB implementation of the transport as well as of the DAC are paramount (garbage in-garbage out), in addition short cable with tight tolerance to USB specs matters greatly, too. In S/PDIF it‘s the originating clock of the transport, the quality and impedance match of the cable and the noise environment around your stereo that matter most.
