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It does sound like the problem is microphonic, though - you could try pressing on these parts with the proverbial chop-stick when the oscillation is happening to see if it makes any difference. (You could do the same, but holding each valve in turn with a thick cloth to see if any of them are causing the problem...)
Good luck...:)
If you're stuck, I can stick some in the post.
Edit: and you can coax better performance from it.
I'll get me coat.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
Lifting the white wire at the left end should kill everything except the reverb signal. Do you still get dry signal through? If you disconnect the reverb tank, do you still get signal through? With reverb on zero??
Lifting the black wire (to pin 7 V2) should kill *ALL* the signal and render the amp non-functional. If you're still getting signal through, it has got to mean a wiring error of some sort...
Lifting the white wire to the reverb pot should kill just the reverb signal, but it also removes the ground reference for V2a grid (there are capacitors in all the other connections), so likely this valve will run away and become saturated due to grid leak current. Likely this is why the amp appears 'eerily quiet'. You could try grounding the other end of this white wire instead, but (if all is correct) this is the same as turning the reverb pot to zero.
Are you disconnecting the input or output of the reverb tank (or both?) - if you have just been disconnecting the input, it it possible that the tank itself is responsible for the microphonic feedback.
I'm looking at this schematic, btw:
http://www.spearfoot.net/sfpr/princeton_reverb_aa1164_schem.gif