"Quieter" fan for whom?
I remember that you have to totally rebuilt the case or housing in which the amp/PC resides in to get a silent fan. Those "owl feather" principle ones may come to your rescue but - by and large - if you have to make them silent, you really have to increase the diameter area, so they don't have to run at same speed, but moves air through a larger hole. Speed is what makes the noise. If you re-house all of your amp into any 3-4 rack space case housing you can use huge fans, and they can run smoothly and shift a lot of air. I would even advice of using two fans, one "intake" and one "out-take" at the back. I e one that sucks in air from the front, passes it through, and one that pushes it out again at the back, or up through the vent holes of any amp head, of course at different directions. Then you'll get the most put through, and can turn down RPMs considerably and make them even more silent.
Use two of these then, BTW they're not Papst, they're called Noctua:
The "owl" principle is detected at the corners of the fan blades, which are jagged and shaped in a very special way, akin to that of an owls wing, hence the owl can fly silently in the night without being detected by its prey even when close up. Cunning solution, by nature and evolution. This principle of fan blades, moves just as much air, and cools the same way, that any of the regular ones, but are way more silent. This of course, with the same diameter, and everything else equal. The material in which they are made of makes significant contribution too, to the resulting fan noise.
The one thing you have to do anyway with these, that sucks IN air from the front, is that you really have to vacuum clean (hoover) it ever so often. They drag in dust too, that sets on the circuit board. Not everything gets sucked or pushed out by the back fan. Maybe a trade off that you don't want to deal with.
Oh yeah, they will cost you some serious dough. Maybe that too, is a trade off you don't want to deal with...
Here's their explanation of their "bevelled" fan blad corners and tips:
Noctua
"Due to the Vortex-Control Notches, trailing edge vortices are split up into several smaller vortices. Thus the noise is spread over a wider range of frequencies and therefore perceived as much less annoying."
