how to value Fisher preamp


I have a Fisher 400-cx-2 in the walnut case, 100% original, never touched and is 100% working condition. It is part of my dad's system that he assembled in the early sixties. He was always very fussy about it and and it shows. He passed many years ago, and mom feels like it is time to sell it off, but we need to get some ides about price and if anything needs to be serviced. There is also a Fisher KM-60 tuner in the walnut cab that works perfectly, stereo beam and all.
Two of the knob trim covers (gold)are off(one missing), but the knobs are still intact. I am wondering about finding the knob cover and if they can be re-glued?
Do vintage pieces like this need to be re-tubed, or if working well just put up for sale, and how does that affect value? Any input is helpful.
rtiff
Having the units updated will risk shipping damage back and forth to the servicer and can take from 3 to 9 months to complete. It probably will not increase the value to a collector. Practical users will not pay as much as collectors anyway. It is a questionable way to spend $500 to 1,000.
Mechans have you evaluated the phono stage on your 400? Im playing with it now and have some mixed feelings....