Another option is insert an active subwoofer crossover to your existing preamp, like the JL Audio CR-1 which featured adjustable crossover point and 12/24dB slopes.
https://www.jlaudio.com/products/cr-1-home-audio-subwoofer-crossovers-96020
https://www.jlaudio.com/products/cr-1-home-audio-subwoofer-crossovers-96020