If you’re going with Paul McGowen’s approach, I think the best way to do it is to grab the signal off the speaker amplifier binding posts, drop the voltage drop, and send it through a DSP and then to the subs. This lets you shape the bass waves which is an audiophile requirement for low bass in small listening rooms.
You may also find that high passing the mains increases overall sound quality, especially at higher output levels.
You may also find that high passing the mains increases overall sound quality, especially at higher output levels.