Being a Magnatone nut, I've mostly thought of varistors being used in Bonham's patented vibrato circuit. I've read that metal oxide varistors are used in some modern amps to provide sort of a "fail safe" for the fuse, from what I can gather. I do also recall having seen one used in one or two of the highly regarded DeArmond/Martin amps but I don't know what they were doing there. Here's another example. The 12AX7 to the left is providing some kind of recovery function after reverb and tremolo. Between there and the phase inverter 12AX7, which is on the right, is one varistor, with no value or rating specified from what I can tell. Any idea what it's doing there?