Since I have repaired many products over the past 20 years, the majority of issues are not from the capacitors but mainly from circuit boards. They become brittle and the solder traces begin to break. So repairing that issue means running jumper wires underneath the boards. Then once those are repaired, other issues seem to pop up. Mostly all this is because of heat and smaller case designs.
As far as replacing parts, you should hear issues as they go bad. Noise, distortion, hum, fuses blowing, etc. I don't think you need to do preventative maintenance. I am not sure if parts quality was better back then versus today, but some parts today are better sounding such as copper capacitors, and some resistors like Vishay, caddock, shinkoh, etc. Most manufacturers do not use high end audio capacitors. You won't open up a component with Duelands or V-Caps in there that is for sure.
As far as finding a good repair tech, that is more trial and error. I have been fair with my repair pricing but it also gives me issues when I repair a component and say the estimate is X and then I find other issues especially with older component repairs. It is also not my main source of income so I can be cheaper with my pricing but it may also take me a little longer.
Happy Listening.
