I got an aftermarked cooler. Not a Papst, which happend to backordered, but some chineese stuff. I took the most quiet I could find - 19dB. It costed 10$ in a nearby computer shop. I also got a manual speed controller, which added 5$ to the bill.
With the speed set to max, the cooler as efective as a PS Audio unit - at least judging from the temp of the housing. Plus, as expected - it is 10x more quiet !
I can stil hear it from my listening possition - but now barely. And when I want it to be DEAD quiet - I set the controller to min speed. At min speed, you can't even hear it at late night listening, with everything switched off in the house !
I estimate, that the air output at min is about 50% of the output you get when you set the controller to the max. The unit gets considerably hotter - but not excessivly so.
And since the the regulator came with 0,5m of cable - I placed it OUTSIDE the P-300 and can controll the fan on the fly.
Now I turn it down every time I listen to some music, and turn it up when I don't.
I couldn't be happier.