Read the Sound Doctor's (
Barry Ober) paper, and watch a few Dennis Foley videos on room acoustics and subs.
http://soundoctor.com/whitepapers/subs.htm
In essence, recordings on LP's deep bass is mono otherwise the needle would jump out of the grooves, and on CD's deep bass is mono because that's the way it is recorded and mixed. The exception is a few Techno-Rave tracks where the bass is supposed to swirl around the nightclub/dance floor (or so Barry says).
The author's of these documents neglect to mention Reel to Reel (Master Tapes) as I suppose they might disrupt their argument.
So bass is mono in the recording, it's directional because of room interfaces, and less directional as you free yourself of adjacent room surfaces.
I set up my room like the above diagram (unknown original source), and it works, it's the only thing that has ever worked for me.
Good video on getting the sub away from room boundaries.
https://www.acousticfields.com/ideal-subwoofer-placement-pressurization/
Lots of reading also lead me to believe the lesser of evils is using Low Line Level IC's inputs from a dual output preamp verses speaker wires to High Level Inputs.
Barry Ober says use a real crossover after the preamp and don't get upset about putting more stuff in your signal chain, if you only knew how much that signal has gone though in the recording process it would not be so precious to you.
Sorry, I got it to work to my satisfaction without another box and more wires.
I would not call my small room listening position "nearfield" however in regards to the subwoofer it is, and it works.
https://whatsbestforum.com/threads/kach22is-system.30259/