Sounds somewhat like a problem I had a couple of years ago (a 2015 Lacquered Tweed BJr, about the same age as yours) that turned out to be a bad screen grid resistor feeding the power tubes. I understand that it's not an entirely unusual failure. I described the problem to my tech like this: A loud pop and no power. Fuse burned. I replaced the fuse and restarted. Low hum (louder than normal), then a pop and a louder hum. It does this consistently now, at varying volumes, every time I start it. Hum, pop, louder hum. It does play, though poor quality. Swapped in new power tubes with no change. Swapped in new preamp tubes. (Did a lot of swapping, actually, different combos of new old.) With all new tubes, amp plays but fluctuates. One moment all is well, but seems that as it warms it up it sounds a bit diminished and breaks up very badly in the lower notes. Almost normal in the upper. Sound familiar? If so, sounds like it's tech time to me. This is not something you want to tackle. Good luck!