Ive worked on a fair few Deluxes and modern Fender amps , A very common fault is damage to either preamp valve sockets where the pins go in or they lift from the pcb where there soldered , almost always when this happened the user tried to change tubes themselves , either missaligned the pins or used to much force and damaged the board in the process , it can be hard to spot too . As a matter of course now I reflow all the tube pin connections on them . If you have a pin thats got a partial intermittant connection ,you could well have arching ,in that case the pin wont really take soldering well until you clean away all the oxidised solder and carbon first . With old bakelite sockets you can often scrape or sand away any carbon deposits ,ceramic sockets that are glazed clean up fine too ,but some un glazed ceramic types hold onto the dirt as the surface is rough .
Carefully inspect all your tube sockets ,wash away any signs of arc with an old tooth brush and isopropyl , if you do happen to find a pin with bad solder ,first use a solder sucker or wick to remove the crud ,then scrape the pin especially around the soldered area as it will have hard crust on the outside due to the arc .Add a little more and better quality solder when you reflow it , should last for years . I have seen it happen as well like in your case that swapping out the tube appears to fix everything ,ocassionally due to a bent pin on the tube it just about makes contact or just about doesnt ,swapping in a new tube with nice straight pins and things can seem fine again for a while .
