In looking at the design of this amp, the heatsinks run just about completely from one side to the other. The fan and intake are located on the rear panel, and the exhaust is in the front. The airflow is blocked so much that a lot of it blows through chassis cracks, around switches, etc. There's actually a very strong stream blowing out the opening for the stereo/bridge/parallel switch! The pic I have isn't the best, but the fan is almost visible on the left just past the warning sticker. Note this was taken when I got it home; it's much cleaner now. There is a bit of space under the main board, as the optional output transformers would be located there, so perhaps some air flows there, but I doubt its much, based on how much leaks out around that switch in the rear. As a comparison my QSC USA 900's have what looks like a cheap bathroom exhaust fan with no shroud mounted in back blowing over 'fingers' of the heatsinks, and it's much quieter.
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QSC's literature states that it's a 2-speed fan, and it's on all the time. If this is low, high must sound like a leaf blower! As it is, my HEPA filter on high is about as loud from a couple feet away. Without knowing the current draw of the fan, what's a good resistor value to start with? If all goes well, this will be use for a subwoofer I'm building. It'll be running bridged into 4 ohms, so there will be a good bit of current draw, I'm betting. It's rated at 900 watts at 4 ohms, and the subwoofer is rated at 1000 watts, so it should be enough power, but I don't know that it'll be a heavy continuous load.
These are 'pro' amps as well, and I did get the fan on my USA 900 to run on high for a while once. Honestly, I was seeing how my speakers and that amp worked together at high levels (around 115dB about 6' away), and the clip lights were flickering; that amp is rated at around 440 watts into a 4 ohm load, which is what I was running. When the fan is on low, it's barely audible a couple feet away and all but inaudible at 6'; on high, I could hear it distinctly. However, I never listen at that level, else I'd be deaf my now! I bet the fans in the amps you were using are more similar in noise level to the one I'm trying to hush up.
On a side note, both models of these amps are basic, heavy, old-school designs. The are built very well and have held up nicely for being 23 years old. I've paid between $50 and $100 for each of these, with the $100 one supposedly an unused spare from a church. Based on the lack of dust inside, that wouldn't surprise me. The other two have seen moderate use to fairly regular commercial use, based on what the sellers told me and amount of crud inside. QSC has surprisingly complete service manuals for these with legible schematics, testing instructions for every function, part descriptions, etc. They actually sound decent, too. I use one regularly to power a subwoofer with a dead plate amp, and it does a much better job than the original amp. I just tried out the one with the noisy fan, and I was impressed with it. A buddy powers his big Martin Logans from his similar QSC amps, and has told me on several occasions they sound much better than any of his previous amps. If one needs a lot of power and isn't bothered by doing a little work to make them quieter, I can't imagine a better deal than use decent quality pro amps. Sure, some are beat to hell, but many aren't, like these.
