I am using the Audio Control Richter Scale III. It is intended for sub/main crossover. It uses a Linkwitz-Riley 24 db/octave crossover circuit. The frequency is changed by changing a small plug-in capacitor. The unit comes with instructions for selecting the capacitance value. There is a bass frequency equalizer and warble tone generator and a calibrated bass microphone to set the frequincy response and level of the sub. I think that Audio Control has discontinued producing these, but several dealers had stock when I last checked. It cost less than $500 new, and I have seen them listed at about half that on the used classifieds.