I have a 26s and I maybe I can help you but first: Is this "problem" something that just happened (all was OK before?) or did you just notice it?
OK, you don't have a schematic, but do you have an Owner's Manual?
I don't think the problem is in the volume potentiometer. It's sealed and of the highest quality. You might try gently rotating the two balance pots back and forth since you don't use them a lot, but they are also sealed. The possible 'switch' devices which MAY be dirty, in order of likelyhood are:
1. The preamp's global gain DIP switches (these are the tiny rocker switches inside the preamp near the front panel.) These should be rocked back and forth (on/off) to make sure they are making good contact (and then of course reset to their original configuration -- unless you want to increase/decrease the main output of your preamp.) There are other DIP's inside to check as well. depending on what kind of input options (phono, balanced) your preamp has.
2. The mono-stereo and the phase switches (you don't use these a lot, so they could be dirty -- gently switch them back and forth a few times)
3. The tape monitor 3 position switch -- gently rotate back and forth.
4. The source selector switch (not likely a problem since you use it all the time ;-)
Don't forget to check your interconnect fittings, and your main output CAMAC adaptors (if you're using single-ended RCA interconnects.)