This is an old but still very actively read thread so thought I'd share.
Quite a while back, I had replaced the 3 speed control with a 50 ohm rheostat which worked very well as an infinitely variable speed controller.
Finally got around to test replacing the 50 ohm rheostat with a 16khz PWM dc controller from AMZ and added an illuminated power off switch to prevent the PWM vampire battery drain when not in use.
This PWM controller is almost as quiet as the rheostat at lowest speed but hopefully will deliver on saving some battery.