Assuming no success there either, power down, and unplug ribbon cable Conn 2. That WILL isolate the Power Amp circuit from the Preamp Circuit, and only contains the rear panel Effects Send/Return jacks, footswitch jack and Power Amp input circuit. Power back up with it disconnected to see if the problem continues or not. If not, then unplug/reconnect both the Conn 1 and Conn 2 ribbon cables. Two main cables.....Conn 1 and Conn 2. Exercise those connections a few times to see if after doing so, anything has changed. If not, then I normally remove the rear panel PCB for close inspection for solder joint fractures. I haven't seen the insides of this amp, so I don't physically know how difficult it is to pull the amp apart for inspection of all the PCB's. You've already been probing with chopsticks without success, which often finds the problem components having intermittent solder joint connections.
Pounding on the chassis to see if the problem is activated by vibration....usually caused by solder joint connections within. It looks like the Reverb circuit is a traditional 'tank', instead of a solid state reverb circuit. I'd disconnect the tank Output from the amp's return circuit to see if it's involved.
If none of these reveal where the problem is, then you're into measurements with test gear to find where the fault is. I'd begin with the power supply circuits. We don't know your skill level in testing/service, as at this point, you're dealing with some high voltages and shock hazards.