What is the serial number on that amp?
If the pan doesn't make crashing sounds when you rock the cabinet back and forth, then there is a problem either in the pan (output section), or in the return portion of the circuit.
If the amp DOES make crashing sounds when you rock the cabinet back and forth (but no reverb), then the problem is likely to be somewhere between the 12AT7 (which should be OK) and the pan input.
Let me know the results of the crashing tests, and the serial number. The serial number will also give me some clues.
I didn't expect Bruce to remember every detail of what had gone before, but having previously stated twice that I couldn't find a serial number, it was frustrating to see that as the first question.
Frustration was to be the overriding theme from henceforth however.
At least there was a sensible suggestion included, but why didn't Bruce or I think of that a year ago?
I tried the suggestion and reported back as soon as I could - see part 14.