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If the sound cut is responsive to the knob like that then it is hard for me to imagine this has to do with a preamp tube. Doesn't mean you shouldn't try it. Doing experiments and seeing what happens is the only way to really know what is happening. But also take the knob off and make sure the collar isn't loose. That would be a giveaway that the contacts on top of the pot can rotate and short the signal against the adjacent pot. Since the pot is in bad shape I would replace it anyway. 85% that is all you need to do.