The USB Audio Class 2 is XMOS based.
It is not galvanically isolated. I know this is sometimes touted as a feature, but really the main concern is will the USB add noise to the target? This is largely a function of how the board is routed.
The USB doesn't affect the noise floor in the dspNexus.
Concerning S/PDIF:
Unlike USB UAC2, S/PDIF has intrinsic jitter issues. This is because the clock is part of the transmission starting at the source, A really good S/PDIF receiver, but now obsolete (WM8804/05) can get the jitter down to about 50ps.
This is still too high, but better than the likely original that might be 300ps or even worse from a TV. This is a limitation with PLL clock recovery. We do jitter attenuation in the SHARC instead and pay a lot of attention to clock circuits and routing.
I would save your money on exotic cables. Sure, its good to have reasonable quality connectors and shielding with the phono cables, maybe $10 versions. Expensive USB cables make even less sense. I would use your extra money on room treatment materials or single malt scotch. Both will likely improve your listening experience more that expensive cables.
If you are a skilled audio equipment designer, you are likely making a lot of small decisions that ultimately affect the overall performance. There really isn't one magic thing that changes everything.
Al Clark