Mike, the best way to resolve all of the sonic problems with that amp is to do what you are proposing....pull the chassis out and make good use of the cab.
Imho, it is not worht hte expense of working on it. That said, it is a heat realated problem.
Some 'cold spray'...I forget the name of the products... that will chill components while they are exhibiting the problem might help locate the problem...might not. I would guess that there is a problem with electrolytics, transistors, resistor, other caps.....long list, right?