Playing last night, I noted, after about fifteen minutes, massive breakup, volume loss, distortion, etc., in channels 1 and 2 along with a momentary power-loss to the whole amp (not long enough to reset the channel back to 1, but enough to dim the LEDs on the amp and the footswitch). As far as I could tell, channel 3 was unaffected.
Looking at the tube chart in the manual, it looks like V3 is the only tube that is exclusively used by channels 1 and 2 but not by channel 3 (on the other hand, what tube is responsible for channel 3's fifth gain stage? It has six stages, but it seems like no tube does stage five). So I'm going to start there. I'm also going to replace the rectifier tube since it seems that has a reputation for being implicated in sudden momentary power losses to the whole amp.
If that doesn't resolve the issue, I'm going to replace all four power tubes, and if I'm still in the weeds, I guess it will come down to slowly swapping out each preamp tube until I isolate the bad one. Assuming it's even a bad tube! If I can't otherwise resolve the issue, Petaluma is only about an hour away by car.