keeping amps cool?

You will skew the setting even on a fixed bias amp.

How does that change the sound, Geoff?

That's right. And some come with one that operates constantly. Some don't come with one at all. Hmmmm.

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.

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.

Any tech with more knowledge than just trouble shooting. Someone who specializes in this kind of work with lots and only good feedback. You have to do your home work, seriously.

I had four Acoustat TNT amps rebuilt and modded by Roy Esposito in Florida recently. He was a principal engineer in that company and instrumental in the amp's design. He now runs a shop dedicated to preserving the Acoustat line. He added balanced inputs and mono capability along with the full rebuild/upgrade. The amps are very impressive. I've had them for about a year now and they just seem to get better and better. They never cease to surprise me. I cannot find fault with them. They have everything in spades. However, he is not cheap. Cost of each amp was about 500 and the work about 900 each. To use his words, "This amp will run circles around amps costing ten to fifteen thousand". He wasn't lying. Imo, he was being conservative. I believe he's willing to take on other brands as well.

In Toronto there's a guy who used to work at Brack's, a high end audio shop, who now runs his own show. Very highly sought after. His name eludes me but I could round it up.