I'd start by swapping tubes. Not the cheapest, but is the easiest.

I had similar issues with a bad solder joint on one of my filter caps. I found it by "chopsticking", which is carefully prodding things inside the amp while it's turned on with something non-conductive, like a wooden chopstick.

I had similar symptoms with a Peavey Bandit when one of the input jacks cracked where it mounted to the PCB.