Someone told me one time, if it ain't broke, don't fix it. I try to follow that with the exception of coupling caps. I think changing the coupling caps can many times improve the sound of the amplifiers to a significant level. But changing the resistors, as I stated, previously destroyed the sound magic. As far as maintenance, the only things I would change are, first, some of the old units used selenium rectrifier bridges. Those were terrible and would fail according to my techie friend. That to me is the only required change I would make. Second, in agreement with the above posts about some of the older electrolytic caps. Finally, I would use a contact cleaner for the potentiometers and clean the grunge off the old RCA inputs. I didnt change them, because as a collector, I wanted them original. But for ease of use, if there was something I would not sell, I would change them. I have nothing left on Scott now but try and get a hold of that old absolute sound series. It was detailed and in some cases, Steve talked, I think, about maintenence. There was also an old Sound Practices or Glass Audio article about how to maintain these old pieces. I dont have these any more so I am sorry I cannot provide much detail.
Good luck.
Good luck.