PrimaLuna tube amp issue

Ah, the joys of ownership of tube gear with high internal voltages! I assume that the main circuit fuse isn't blown as you still have a working channel. Unplug the amp and let sit overnight. Then remove the bottom cover and take a look inside. Look for a blown cap with stuff coming out of one end. Could be a blown coupling cap. If so replacing it may not solve the problem! I wish all tube gear manufacturers would provide schematics to make troubleshooting easier! 

Look also for burnt resistors on the circuit board. 

I would reach out to Upscale Audio for support. 

Maybe swap the LHS with the RHS speaker wires.
If the bad channel moves then swap the interconnects LHS with RHS.

If the bad channel stays put, then it is you speaker.

All tubes including signal tubes? or Just the power tubes?

Are the valves lighting up? If they are not lighting up, that's one thing. If they are I'd be looking to the small valves.

Speaker wire swap to check for speakers.

RCA/XLR swap to check for cables and source.

Not a fuse in the speaker?

Not a fuse per channel?

Side Note: It was shipped to your Island.
Tubes 101 IF the valve survives transportation, it should be checked, WHY? That is when they WILL fail, most often after being shipped. Small valves???

Just thinkin' out loud.

Happy hunting