Coax SP/DIF to USB..thoughts?

USB is a relatively new requiment. It can with the need to connect computers to audio systems. Before this SP/DIFF was the connection. USB presents a completely different input to the DAC… the DAC must retime the bitstream. Which sounds better depends on your DAC and streamer. In general, if your streamer is better than your DAC then USB might sound better, and visa versa. Some DACs were produced specifically to be connected to computers and hence only have USB inputs. But of course some streamers then started to have USB outputs.

For contemporary and equal quality streamers and DACs SP/DIFF (or AES.. the balanced equivalent) is best. But you have to try both to be absolutely sure. I have very high quality equipment, there is really no substsncisl difference between the two, so I use the AES.

Avoid conversion. 

OP,

Yes my Aurender has all three… I use the AES.
 

Actually it has lots of additional outputs… even dual AES. 

My experience is that ultimately the sound quality is about 50-50. While at lower levels the DAC makes a bigger different as the quality goes up the difference equalizes and really high end sound requires both at the very highest level. My streamer is actually more expensive than my DAC.