A variable power supply would definitely give you more control and with enough current allow you to run a chain of fans at lower than normal speeds. Ideally you would want the largest DC fan you can get and run it below its rated voltage. The larger fan running at a lowe speed will typically produce less noise than several smaller fans. More importantly is that you will have fewer fans on your equipment. As an example, you can look at Mouser Electronics (http://www.mouser.com/catalog/622/1316.pdf) where you would choose the largest fan with the lowest operating RPM.
It is true that heat breaks down semiconductors. Capacitors also do not like heat. I had to repair a friends NAD cd player that had burnt out semiconductors. The replaced parts also ran hot. That was about four years ago and the unit is still in operation.
A fan is a good idea. Period.
