Do you have a schematic for this amplifier? I was looking at the Pro Reverb schematic at the Fender web site and see that this is an all tube amp including the reverb drive and recovery circuits with an 8 ohm input tank, nice. From what you have posted the reverb will start to feedback with no signal into the amplifier just turn up the reverb level pot. By looking at the schematic for the Pro Reverb if the signal level out of the reverb is high enough it could feedback across R76 to its input and then oscillate (squeal). This is probably at a frequency greater then you need for the range of the guitar. One way to prevent this would be to limit the bandwidth of the recovery amplifier the 12AX7 V5-A. If you add a 300pf 500volt cap with 10k ohms in series across the plate load resistor R71 this will roll off the maximum frequency from the recovery at approx 5 kHz and may stop the oscillation. Because this oscillation happens with no mechanical excitation (no input to amplifier) I kinda don’t think it is a microphonic tube that is the cause.