Is your sub powered? Many subs have a crossover built in. It might be cheaper for you to buy a new sub. Part-express sells subwoofer crossover in a variety of price ranges.
Best Way to Split Amplifier Output Frequencies to Main Speakers and Subwoofer
Is there a decent stand alone active or passive stereo frequency divider that can take the speaker outputs from my vintage receiver and send mids/highs to my main speakers and lows to my subwoofer? I want to relieve my main speakers from deep bass responsibilities. The crossover frequency should be user adjustable. Neither my receiver or sub have the capability to do this.