My problem **was** in fact with USB implementation: brief interruptions of play ("skipping"), usually once a track.
By luck, after 8 months of headaches, I figured out it was problem involving the interaction between the PWD and my MACbook (it did not occur when I swapped in another DAC, nor did it occur when I used a CDP as transport).
Both the MAC "geniuses" and PSA disclaimed knowledge of how to resolve the problem, and responsibility for doing so.
A random software update after I'd decided to give up on PSA resolved the problem. I kept the PWD, but the experience did very little to improve my mood.
Best,
John