I just put a PWM speed controller in mine. I've never hooked it up with the standard 3 speed controller to compare.
http://www.ebay.com/itm/221325292956?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649Works well, except at very lowest setting where there's a high pitched whine(typical of PWM) Fitting it in was a challenge and ended up removing one of the screw posts with a dremel to get it to fit inside.
If you go the resistor route, I'd cut the heaviest resistor coil on the 3 speed controller, and wire a potentiometer in it's place.
Make another hole and knob to mount it. When the 3 speed is on setting 1, you can use the potentiometer to adjust the speed.
On setting 2 and 3 it just works like before.
You'd have to measure the resistance of the coil you cut to select the right pot rating. Go higher resistance than what you measure and high enough amperage rating.