So my amp(crate vintage club 50) has been rather strange lately. I can play it for about half an hour and theres no problem at all. Than i start to get that super saturated dieing battery sound while the volume goes up and down. I was told it may be the power tubes and figured i'd change them but no dice on the solution.
So you have changed the power tubes? If so, go buy a 12AX7 and try it in each position.