I replaced the stock Antec fans with 2 Gelid Silent 12 PWM fans. I was very happy with the results. Because they were PWM they operated perfectly with the Antec ChillControl V software - in terms of controlling fan speed. These fans idle at about 750 and max out at 1500 rpm. They were MUCH quieter at the "Extreme" settings with respectable temps of about 62º (Liquid temps of 33.8º) when running Prime95.
I was finally able to get 2 Scythe Gentle Typhoon AP-15's this week (excellent price $13 plus shipping) and replaced the Silent 12's. Temps are better with my hottest core at 59º (Liquid temps of 31.7º) in Prime95. That's a 3 degree drop...the downside is that since the AP-15's are not PWM, they are running at max 1850 rpm all the time. They are quieter than the Silent 12's at max rpm, but I only ran the Silent 12's in Extreme when benchmarking etc. The other 99% of the time they idled nicely and were much quieter than the AP'15's, which produce a constant low pitched hum - not too annoying, but the idea was to get as quiet as possible with max cooling for the Antec 920.
Unless I can get a fan controller like a Lamptron FC 2 that will allow me to dial back the noise of the AP'15's, I might just re-install the Silent 12's. Any other ideas how to control the AP-15 fans when not benchmarking etc?