Hey fellas...in another thread, johnplayerson shared an ebay link to a seller offering $5 digital low voltage controllers.
They are small and can be placed in-line with your stock fans...or replacement fans. If you wanted to experiment with reducing the voltage a little bit to cut rpms of fan, and thus noise, this would be the way to do it, especially for amps that don't have more sophisticated and variable cooling management.
When I used to build PCs 15-25 years ago, I would use just such a device (except it was a mechanical knob at that time, not a digital control board) to control case fan speeds because a lot of mother boards did not have effective, or any, control through their fan headers.
At that point, it was a matter of seeing how much I could dial back fan speed while still maintaining an acceptable thermal envelope with the CPU during the most intensive activities the PC was going to be subjected to on a regular basis.
As far as replacement fans, as long as specs such as size, voltage, amperage, and cfm are a match (cfm could exceed original spec of course!), just find a real guiet ball bearing or fluid dynamic bearing (FDB) fan. Sometimes you might have to monkey with connection headers (2-wire, 3, etc).
In counterpoint, the amp designers obviously speced this fan...but, if we are using these amps for a different duty than was intended in our home theaters, maybe the speced fans and fan speeds can be modified slightly?????
Here is the link to the AVS Forum page where this is being discussed:
The control board is at top of page and eBay link is closer to bottom of page.