I think three ways a streamer can impact sound quality:
- USB / Coax ground loops
- Excess jitter
- AC line noise
I think paying attention to the quality of the wall wart (if used) and making sure the USB cable is isolated (when possible, not all DAC's like this) takes care of most. At the end of the day though your DAC is going to be subject to the clock of the streamer, so having a streamer with a good buffer and low phase noise clock is important.