Your volume and balance pots can't be replaced because you've got the wrong tech working for you. Find a better tech.
It was not just my tech who told me the identical Luxman volume/balance could not be replaced. A business known for doing vintage restoration, Hudson Valley HiFi, told me the same thing. L507 Restoration by Hudson Valley HiFi
They did say that I might be able to eliminate the balance pot and look for a compatable replacement, but it would not be easy or inexpensive.
Thanks for the Khozmo link. They look to have some high quality alternatives.