I'll go with Elizabeth on this one. The only problem with fans is the noise and perhaps the added complexity. Heat sinks are big, heavy and expensive. Commercial amps virtually always use fans for all the above reasons. Krell used fans for quite a while. A cool amp will always last longer. Temperature cycling is no good for anything. Electrical conductivity decreases with temperature which is why the fastest computers do crazy things like run in refrigerated freon baths. They do all kinds of stuff to keep superconductors cold. I have no idea how all this pertains to the performance of relatively mundane devices like audio amplifiers but I use a fan cooling system on my amps. I run a 12 volt power supply off a switched AC outlet. The switch is right next to my equipment cabinet. The supply triggers all the amps and runs #4 6" fans one for each side of the two main amps. I put a 100 ohm linear pot in series with the + side of the fans so I can control the speed (noise). It all works brilliantly. Whenever I turn the amps on the fans light up automatically. The whole mess is mounted on a shelf directly under the speakers in the basement. I hooked up the pots as an afterthought. I could hear the fans running full tilt under the floor! Backing them off a bit worked fine.
Oh, and if you put your amps on the floor make darn sure they are not on carpet, any type of carpet. Hard surfaces only. Any carpet will interfere with the flow of air over the heat sinks. I set up this fellows system years back. About a year later he called me up all bent out of shape because his amp died. The genius had his room redecorated and he put the amp back down on carpet. He did not know that it would take some time for it to reset itself and thought it has died permanently. An amp stand solved that problem.
Oh, and if you put your amps on the floor make darn sure they are not on carpet, any type of carpet. Hard surfaces only. Any carpet will interfere with the flow of air over the heat sinks. I set up this fellows system years back. About a year later he called me up all bent out of shape because his amp died. The genius had his room redecorated and he put the amp back down on carpet. He did not know that it would take some time for it to reset itself and thought it has died permanently. An amp stand solved that problem.