hmm a project. give me a day I will let you know what I come up with.
I never had one go bad and never replaced one.
But I do have a few ideas.
I have about 6 spare apw3+ to test.
what will be the power draw 400 watts 500 watts?
https://www.delta-fan.com/catalogsearch/advanced/result/?fan_type=19&p=4&q=1&voltage_rated=5701afb0612h-a
afb0612h-af00
are 2 pin and 3 pin they do 4250 rpm not 6000
my guess is they will do the trick if you don't push the psu
afb0612hh-a 2 pin 5000 rpm
afb0612hh-af00 3 pin 5000 rpm may be too loud
i have to think the ones above will all work at 500 watts or less.
there is a 3000 rpm that will be quiet but not sure if t will cool the psu off.
afb0612l-a 2 pin
afb0612l-af00 3 pin
If I was doing under 500 watts I would guess the 4250 rpm
if I was going super low 300 watts maybe the 3100 rpm