I have a number of AC Infinity fans in my Sanus rack to help with cooling. The rack sits in a closet that's on a platform even with my riser in the back left corner of the theater. At the very bottom of the rack is my Parasound A21+ two channel amp which drives my B&W CM9 speakers. They like a lot of power so the Parasound warms up pretty quick while driving them.
Right above the Parasound I have a 1U AC Infinity Cloudplate intake fan. This pulls cold air from the lower front of the rack over the Parasound. I run the AC Infinity in "smart" mode, so it kicks on at low speed around 82 degrees and I rarely see it get above 86-87 when pressing the amp hard.
My Marantz pre/pro also runs warm so I have an AC Infinity T8 that sits directly on top and vents to the back of the closet.
Finally at the very top of my rack I have an AC Infinity Cloudplate exhaust fan that moves the hot air moving to the top of the closet out of the rack. Additionally my air return for the room sits right in front of the rack so it helps pull the hot air out of the theater.
The Cloudplates do get loud if you start to push the higher fan speeds which is really my only complaint about them. I'm not sure if the 2U and 3U models are any quieter or not but overall I'm happy with how they work.