I swapped the two 24v 80mm fans (two wires) in my cdi 4/1200 bl with low-speed ones with great success.
I recommend fans that spin at around 2200 rpm (about 27 dB) instead of the 1500 rpm (17 dB) that I bought since my channels are now running at around 68 degrees.
It's dead easy to change the fans, so no worries.