Replace volume pot

Most likely it won't do anything for you because pot's are used as dividers and not resistors. They select value around 20k - 100k because smaller values tend to load previous stage while unnecessarily large pots are prone to noise pickup and might create a low pass filter with input capacitance of the next stage.

Marakanetz, 50k and 100k pot set at exactly same angle provide almost exactly same loudness since its operation is ratiometric - resistance have nothing to do with it, I used word "almost" because there is not two exactly same pots, even with the same value. Value is roughly selected by the reasons I mentioned before. One designer might select 50k while another might select 100k since it is not that important. It does not change gain.

Marakanetz, It looks like it is not straightforward but rather some convoluted combination of Volume and Balance with addition of Phono. I wouldn't touch it with a stick.

As for technicians - it is different (from designer) set of skills. Technician should look for faulty or misplaced component or open/short circuits but should not modify circuit under any circumstances (worst kind of technicians) since it is designer's job. Designers on the other hand don't have enough troubleshooting experience to find problems efficiently and to apply proper fix. So, when you talk to techs about circuit modification remember that they don't do it (or at least shouldn't).

Marakanetz, I can attempt to help but have to see schematics - at least of this section. I built my first amp (100W tube 2xEL34) when I was about 15. Later when electronics became my profession I lost interest and now prefer buying instead of building. I spent best half of a day inside. Sometimes I thing work outside would be better for me (in spite of money).