Your system suggests you would appreciate the sophistication of an OB/Dipole sub. A very unique one is available as a joint venture between GR Research and Rythmik Audio. The sub is not a normal Open Baffle sub, which are already unique enough, but an OB in an H-Frame, superior to a flat baffle because of it’s greater structural stiffness and non-resonance. And of course it’s open baffle design prevents the often boomy sound of even the best sealed subs.
A dipole sub excites the room modes of it’s front and back output, but not those of the room dimension to it’s sides, where there is a dipole null. There are a few OB/Dipole subs available, but the GR Research/Rythmik is unique in that the two (or three) 12" woofers (facing in opposite directions in the H-Frame) are Servo-controlled by the Rythmik plate amp. The amp also contains a 6dB/octave shelving circuit to offset the dipole front-to-back cancellation at very low frequencies.
The sub is offered as a kit only (one kit includes the amp and two---normally, though you can use three---12" woofers). The H-frame into which the kit is installed is available as an easily-assembled flat pack from a couple of wood workers who frequent the AudioCircle GR Research Forum. A pair of kits (for two subs) runs about $1500 shipped, and the flat pack (CNC cut from 1.5" MDF!) about $600. The highest sound quality, though not quantity, sub available.
Details of the design available on the GR Research and Rythmik Audio websites, and discussion of the sub on the Forum.