Since you have brought this up a few times already, I think you should have the caps replaced mainly for the peace of mind you will have.
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Rocky - The real problem is that they can fail suddenly and without warning.
So, the real question is, what would happen if they did? Can you find the replacement parts if they take out a bunch of output transistors, and would you afford it?
In theory, they could last another 20 years. They could also blow and take out a few parts. When that happens 100% you'll end up replacing all the electrolytics anyway, soooo....
They simply dry out. Dry capacitor has increased ESR (equivalent series resistance) resulting in poor control of the bass.
When ESR gets high capacitor gets hot from the current flowing thru it. Increased temperature increases ESR even more and eventually it goes into thermal runaway (avalanche effect). Before it happens though, capacitor will be at least very warm. If it is cold and sound is OK then leave it. Life of capacitor is in order of 50 years in normal ambient temperature, but each 10degC temp. increase cuts life of capacitor by 2.