OK, a brief little electrical engineering explanation is about to happen....
Just using a rheostat (a potentiometer) to control the speed of the fan by increasing resistance, thus decreasing the current you can draw for the fan, does not work....its messy, and introduces noise into the line.
Rather, get a controller (like the Vantec ones) that take in the 12V line, and use a single LM317 Voltage regulator for each fan. These allow you to alter the voltage to whatever you like, while having capacitors in there to clean up any line noise (LM317s are not prone to create line noise, unless the noise is already in there from the PSU.)
Make sure you know what you are buying I'd say....if it is a simple POT, don't touch it....you could make one for $.30........