If you are for sure that your tubes are not bad and the socket which holds these tubes are check and inpected then your answer would be:
There could be two things.
1) Your bias pot are going bad. Clean and do measurement on both channel. See if you get a reading. Turning clock wise or counter clock wise will either increase or decrease. If both has the same reading ( you are good).
Next step.
2) one of the resistors to ground could be open or bad.
There could be two things.
1) Your bias pot are going bad. Clean and do measurement on both channel. See if you get a reading. Turning clock wise or counter clock wise will either increase or decrease. If both has the same reading ( you are good).
Next step.
2) one of the resistors to ground could be open or bad.