Is this Rob? Thanks for the input.If you are sure you're in USB mode, my next suggestion would be to try a different rectifier tube. Even if the other tubes are lit, a failing rectifier can cause you to have no sound in either channel.
I did swap out the rectifier but even though I get high voltage at some of the tube pins I still have no output on either channel. The USB button is pushed in and Roon recognizes the B3 and acts as if music is playing so I assume that the USB board is on and working. Looks like a warranty return...