I recently put together a new Haswell system (i5 4670, Asus Gryphon Z87), my first since my i5 750 system four years ago. The latter used a Scythe PWM fan on a Hyper 212 Plus, where it ran around 350 RPM at idle and was essentially silent. Very nice fan. They aren't available anymore, and Scythe seems pretty much gone altogether, so I'm wondering what to replace it with. I've moved the Scythe to my new system, as it's my best PWM fan, and on its Hyper 212 Evo, it runs about 180 RPM (!) on idle. Asus's "quiet" mode is quieter for the new mobo. It's also hotter, but still totally acceptable. BTW, it's controlling my P182's rear and lower chamber Nexus fans perfectly, idling them around 580 RPM with their respective FanMates turned up to max.

I have Gelid, Cooler Master, and Akasa PWM fans, and the Akasa Apache is the only one that will run below 600 RPM on the old system, where the Scythe ran at 350. The Apache runs at 500 RPM, which is pretty quiet but not as quiet as the Scythe was. The Apache is the best out of these three.

I would recomend the Arctic Cooling F12 PWM. I have a couple for a few years now and have no issues. They can drop to 150-200RPM, waaaay below the "imposible to hear with the ear next them" level of ~350 RPM.

I recommend Arctic Cooling's PWM fans as well. My Nexus D12SL-12PWMs ran at ~800-2000RPM stock until I grounded the PWM wires and added a resistor to the +12v line of the faster one. Now they both run ~550-590 but that's no match for the Arctic if you're looking to easily achieve low RPM while using a temperature-sensitive controller and a PWM fan.

That AC fan looks like a good bet. I wish it didn't have that extra header and RPM sensor cable, but I guess I can cable-tie them out of the way. Anyone know if Scythe is ever coming back, or are they gone forever?