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.