I'm just posting to verify. When playing my marshall it sometimes just goes very quiet in volume. It will then kick back into normal volume later. I'm thinking it's a bad preamp tube, I did notice that one tube is not getting as warm as the others when it goes quiet. Would the best bet just be to swap out one preamp tube and just try it out and see if that does it, or is there other stuff that can cause it to drop in volume randomly?