I don't know how useful is this schematic. I designed two guitar amp with power scaling.....that is adjust the +B voltage from high voltage to low. I never have problem of burning anything. Most likely the MOSFET oscillates and burn itself out. It is very common for MOSFET if you don't have gate stop resistor and careful layout. In my amp, I use 8 position switch instead of continuous variable. But you can easily use a zener to set the voltage and use a volume pot that can take high voltage to adjust the voltage driving the gate. In my circuit, I also adjust the grid bias proportion to the +B to keep the class AB amp with constant bias current. In my circuit, I put a 180ohm resistor in series with the source of the MOSFET, this is to mimic tube rectifier and make the rail not as stiff as SS rectifier to give it some "sag" that is good for guitar amps. You can put lower value if you want stiff power supply.