Here's what I would do.
#1 Familiarize yourself with the fact that voltages inside tube amps can be lethal and how to avoid killing yourself.
#2 Familiarize yourself with the fact that tubes get really hot and you will burn yourself if you grab a hot one without nice thick gloves on.
#3 With the power off, pull out the reverb return tube 6GWB. Looks like it might be the driver also? Turn the amp back on. You won't get any reverb but you shouldn't get any "feedback" either.
#4 With the power off, put the reverb tube back in and disconnect the reverb unit. Does it have cables that go to the rest of the circuitry with an RCA connector? If so, make a short circuit to go into the reverb return jack by taking an old RCA cable and cutting it in half, stripping the wires and twisting them together. Plug that into the reverb return jack. Turn the power back on. With the input shorted, you shouldn't be hearing any reverb (same as engaging the reverb kill switch, more or less).
#5, see if there are any markings indicating "in" and "out" on the reverb unit itself. Possibly they are swapped? (I don't really know how much it matters, but it might).
Report on your findings. Then hopefully one of the experienced tube techs will weigh in with further troubleshooting strategy.
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