Just have it repaired. There is no good way to predict ahead of time what will go next.
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The big item on amps that needs to be thought about are all the power supply caps. That's what will wear after 20 years or longer (depending on when made, new amps have better caps). Power supply caps can go and cause a cascade of failures which can end up turning an amp into a brick.
So, if your amp is reaching that age, recapping is good. The other parts are going to be much more reliable.
Agreed with Ralph and the others on replacing electrolytics after a given age. I have had many successes with my gear with a magnifying class looking at caps for signs of heat, puffiness, or juice leaking out. Since I have a soldering iron and mouser account, I simply replace all the caps if one fails--not very expensive for me. I guess 20-30 years. My limited experience is that mouser and digi key are superb but mouser seems to stock more of the older type caps used by older devices.