It is possible that some very fine wires in the speaker cables have come loose on the + and - terminals causing them to touch each other and partiall "short out" the signal to the speaker. If your close vision is not top notch, use a bright lignt and a magnifying glass to check. I actually solved an intermittant problem in a church PA system that had gone on for years cutting in and out as the heat protection circuits in the power amp was triggered and then, when the amp cooled a bit, the amp came back on. (With a tube amp, that probably isn't happening.) The problem was a sloppy soldering job in a phone plug carrying sound to one of the speakers. I hope you find the problem.
Speaker cutting out intermittently
Primaluna Evo 100 feeding Primaluna Dialogue Premium feeding Klipsch Cornwalls. After listening for several hours while straightening up the house, I noticed change in volume. I checked and found left channel silent. Turned off amp and muted preamp. After a cool down, I reversed speaker connections on the back of the amp, and powered up. Everything good. I gently wiggled them before shut down, with no effect. Since preamp remained on/muted, I’m ruling that out. Power tubes are nearly new KT 150’s. Warning lights for those tubes remain off/good. Changed sources at beginning of this to rule that out. So…any thoughts on this? I’m not a tech guy,,basics only, so any insight is greatly appreciated.