Right channel phono stage tube goes noisy

I have a certain phono preamp, which shall remain nameless, that has exhibited a strange issue on the right channel. Of any brand new tested 6922 tube pair that I insert into the moving coil stage, the right channel plays quietly for about three hours, after which time I start hearing soft popping or crackling sounds that are audible from my listening chair. When I swap the left and right channel tubes, the noise goes to the left channel. Four good tubes have done this. The original MC tube played fine for about six months. I believe something in the circuitry is defective or has gone out of spec or otherwise has sort of failed and is maybe overdriving that tube. I sent the preamp to the dealer for repair. Some sort of modification was made but the problem was not fixed. They tested the first three tubes and they claim that they are fine. Has anyone 1) encountered such a problem, and 2) what was the fix? Was it simply a resistor or capacitor that had gone bad?