Visual inspections show no sign of burnt components, and quick testing of any diodes don't show anything open or short, resistors around the power tubes for bias, and input show no deviation from their resistance values, and a test of the output transformer shows no sign of shorting.

The output impedance is switchable from 4, 8 and 16 ohms, but the 2 outputs are parallel, and according to the manual, the cabinet speakers have to have a resistance load of 8 ohms min per speaker within the cabinet, with the amp switch to 4 ohms, but I later found out that the guitarist was using 2x4 ohm speakers, and using the equation for parallel resistance, the head should be switched to 2 ohms, but the head doesn't have a 2 ohm setting, and reverse parallel equation shows that if the head is switched to 4 ohm, then the cabinet speakers must be 8 ohms each.

The equation I used is:-

RTotal = 1/((1/R1)+(1/R2))

giving that R1 and R2 both equal 4ohms, resulting in RTotal = 2ohms

In theory the mismatched impedance between speaker load and head output could have done something to the power output stage(s), but I'm stuck as to what.

I have exhausted everything I can think of and test, maybe someone can think of something I haven't.