You are thinking a MOV would protect the amp from double mains voltage? Good luck.
The MOV is used to protect a circuit from transient voltage spikes, it is not a power zener that will hold 240 down to 120. If someone switches the amp to 120 while plugged into 240, no MOV will help.
The way to protect your amp from this is to keep the monkeys who might do this away from your amp.