I hear ya... I'm just going by my Marshal days which was quite a ways back. When one tube would go, the amp would start doing what you're describing. It sounds like your amp has "this tube is shot" indicators. Mine didn't. Usually when one tube went, it was time to replace them all, though that was more of a practical decision in trying to avoid having tube failure during a gig. Maybe try getting a replacement, switch out the tubes one by one until you find the bad one? Not sure if that's recommended though.