I bought a SuperPID speed controller and didn't fit it to control my Kress router as I had to modify the router to take out the inboard speed controller and fit the optical rpm feedback.
I finally got to do it when the first set of brushes went.

I wish I had done it sooner.

Speed is now controlled by Mach3 via PWM and the Kress runs much quieter. Also the lower speeds (still at full power) make better cuts.

http://www.vhipe.com/product-private/SuperPID-Home.htm