Just a quick thought Martin....Do you have a pair of any of the high speed fans that you expect to "tick" when undervolted? I ask because of my experience of running two Deltas in series and how much quieter (and of a more pleasing quality) that was than when running on a fan controller.

I wonder if there is any difference on the Deltas or the Panaflo NMB-MAT FBA12G12U for example, between using a fan controller and a simple rheostat?