I'd try swapping the powerstage. For a short test, the swapped powerstage doesn't need to be bolted down.
Note: the powerstage is 2 parts, a black electronics module which is bolted onto an aluminum heatsink. Don't separate them - unbolt the heatsink from the fender instead. If you separate the 2 parts, you need to add a layer of white thermal compound between them on reassembly.
Swapped, didn't make a difference.
Does anyone know where it is grounded?