The "haunted" cabinet has a speaker with a rubbing voice coil.
Speakers can be excited by external sources of
acoustic vibration; in fact, a speaker is a symmetrical
device: you can put AC in and get motion (sound) out,
and you can put motion (sound) in and get AC out, i.e.,
a speaker will also function as a microphone. Since
the cone in the defective speaker responds to the
compression/rarefaction produced in the atmosphere
by a nearby loud sound source, the (defective) voice
coil will rub the pole piece when the cone is vibrated.
The main clue here is that you said it sounds like "fuzz."
That points directly at a rubbing voice coil. I've seen
this many, many times; in fact, in multiple-speaker
combo amps, when someone blows a speaker they
usually disconnect it to finish the gig...but they find out
that they can still hear the VC rubbing anyway. You can
sometimes kill this effect (or lessen it considerably)
by shorting the terminals of the offending driver
together. You can test for it by removing the back
of the cabinet (Kustoms were rear-loaded back then,
and didn't have removable grill cloth) and having
your brother play while you stick your thumb against
each cone in turn; when you hit the bad one, the
"fuzz" will either stop or change radically in character.
You can also plug a speaker cord into the cab's jack
and throw a short across it with a pair of pliers, but
this might not work with three speakers in parallel.
Lord Valve
Expert (please obsess)