Usually a problem with a tube amp would be in a bad tube. If the clean channel works, then it can't be a power tube, since the power tubes run both clean and dirty channels.
I'd pull each preamp tube and replace them one at a time with a known good preamp tube. It's slow going, but that's the best way to know.
Alternative: You could try tapping each preamp tube with a chopstick to check for issues, the V1 will tap louder as it's amplified by following tubes. A bad tube will often show evidence of microphonics and do weird things when you tap it. But the best way is to pull each one, one at a time and replace with a known good tube. Then see if that solves the problem. If not, put the original tube back, and move on to the next tube.
If these tests don't change anything, it's time to call PRS.
