No solutions for your 66' Deluxe Reverb but some funny anecdotal experiences with fellow musicians tube amps:

A friend had an amp, Fender Hot Rod Deville, 4x10, MIM, that would intermittently fade and cut out, and also make popping noises. Changed tubes, re-biased, tightened the tube sockets, and it had already been re-capped with Nichicons. Problem still persisted, especially when played hard, Yet, other times it would work just fine. Go figure?

So I offered take it home and take a look at it. Upon removing the chassis, my nose led me to the problem, because there was a burnt smell. Turns out the ground wire (black) on the speaker jack had the strands of wire tinned with solder, and properly bent in a hook through the lug of the speaker jack, but it was not soldered to the lug! It would get hot when played hard and eventually began to arc. The arcing was the popping we heard, and the cutting in and out was due to the poor connection getting hot. It also arced the plate pins on the 6L6 tubes so we had to clean them, and the tube socket pins and re-tension the tube socket pins. It has never worked better, and was a simple problem that had originated at the factory.

Prior to finding this persistent problem, a bunch of experts had themselves convinced it was a bad output transformer or a bad output tube or rectifier. Lesson learned was that a simple inspection of the chassis, components, and wiring will often reveal the problems. Tubes amps are so simple and basic that it is not hard to smell and visually see problems, especially when they cause a stink.

My other friend ( I know, he is my only other friend), Mark said this the other day as his tube 65' Princeton began to fade in and out while we were jamming: " Hey, tube amps have features!" Turns out it was one of his pedals on his board.

My Fender Bassman would drop off in volume then jump back up, and pop intermittently, very annoying when gigging: it was the input jack itself was corroded inside, the contacts were corroded and not making solid connection to my guitar plug, so the jack was replaced.

Take that jewel to a good tech and he will get it singing again in a jiffy. Good Luck!
Jay