keeping amps cool?

Unless it operates at the temperature it was designed to, it will definitely change the sound. Capacitors typically never get anywhere near their highest allowable temp. Those parts that are vulnerable to thermal breakdown are the ones on a heat sink. If your amp is functional after 20 years, it still needs to be rebuilt.

Why does it need rebuilding if it still seems to function fine?

That's just it. It 'seems' to function. Although it may sound okay, it has by then fallen probably far below spec. If you owned it since new, you probably didn't notice it's deterioration over the years. If it's a great amp worthy of a good overhaul, you're in for a treat. It will likely sound better than new due to current technology parts and maybe a mod or two by a respected tech or engineer.

I use a 24v muffin fan run at 12v and set it on my Mac c2200 tube preamp wired to a door switch that turns on when in use. Works well sucking heat from within chassis.

Ptss,
I would agree with Csontos, and suggest to seriously consider mods such as replacing and upgrading with better quality capacitors, internal wires , resistors, diodes and rectifiers , binding posts ,etc. Actually they do not cost so much ( compared to buying a new amp) but improvement in performance will be outstanding. I did it on my Cary Audio (AES SuperAmp AE-25 ) and thoroughly satisfied , even people at Cary were surprised at the good result.
Reg cooling : I had on the same amp custom installed a 3" cooling fan at the bottom to take air from underneath and throw into the body of amp. I had even a 2 speed regulator installed which make fan to run at higher speed ( more noise /dB ) while amp on stand bye , but run at half speed while amp in operate mode ( very low noise/dB ) . It works great !