[grc] A bit of trouble...

[Paul Bame]

Great detail in this report!

My guess is that there was both 60Hz hum from the usual causes -- poor
ground or no ground or ground loop(s), and with the usual cures...

...AND that your RF was getting into the amp. Look at an amp like a big
radio receiver and /all/ the wires connected to it as antennas -- not
that simple really but a good model anyway.

Based on the report of touching the power cord and buzz changing,
my guess is the power cord was the primary antenna /that/ time. I'd
obtain a bunch of clamp-on or snap-on ferrite cores and attach
them to the power cord at the point where it enters the amp. If
that doesn't fix it, put em on the instrument cord too -- nearest
to the amp as you can get. More cores work better than one. Break
the clips on the cores and use tape so you can re-use them for
the next band. Maybe borrow a guitar amp for experiments.
https://www.amazon.com/RF-Choke-31500-Ferrite-Material/dp/B00XDACVOM