A lot depends on how the DAC handles clock. If the DAC has its own USB interface, it could be operating purely asychronously and using a free-running clock for the DAC chip. Audio quality will not be dependent on clock jitter, because there will be little, if any.
If the DAC uses an S/PDIF input (AES/EBU, TOSlink, and coax S/PDIF are all the same – use whichever you like best – I prefer TOSlink for isolation) then the clock must be derived from the incoming stream. A good DAC, like the BADA Alpha DAC, will use a PLL to lock its own internal low-phase-noise clock to the incoming data stream. No loss of quality. And adding an external USB-to-S/PDIF converter won’t make it any better – or worse.
If the DAC extracts the clock from the data stream without regenerating it (PLL + local oscillator) then it is possible that the quality of the clock in the S/PDIF could have an effect on the sound. I just don’t know how to find out how the DAC processes the clock. If the DAC has two separate clock oscillators then it probably phase locks a local clock to reduce jitter.
I am currently playing with the Breeze Audio DACs available on eBay for $60. They look like they have promise. At $60, what have you got to lose? They go up to 192/24.