Gain pots are shot

If you have the old pots then the value is usually printed on the side of the pot. If there is no number and you have the pot then you can open them up and using a ohm meter take a reading directly off of the carbon film of the pot. Read from one end to the other. I would expect something on the order of 25K (K=thousand) to about 50K. If you can , get the audio taper style of pot, but in a pinch the linear taper will work. The trickiest part will be to get one that is close enough in size and shape to directly replace the original mounting. Good luck. By the way, in extreme cases I've even used fixed resistors (3 or 6 resistors depending on mono or stereo) to replace a pot and then you just choose resistors that will place the volumn at say 2/3 full.