This is so with an AC motor. With a DC motor ( a motor with brushes like a portable drill ) altering the voltage will change the speed. I dont know what type of motor your router has without looking at the specifications.Hi:
A soft-start module is a speed control that modifies the voltage waveform to the motor. Adding a speed control module in series with the soft-start would most likely destory the soft-start, or at least prevent the soft-start from working. The speed controller and the soft-start module both expect a sine wave (remember trig in school?) on their inputs. The output of either module is a sine wave with chunks notched out.