1- If you have the room, the bass panels of any of the Magneplanar Tympani loudspeakers. Secure them to the walls or ceiling for bass down to 30Hz. LOTS of current required. Satie on the Planar Speaker Asylum powers his with 2,000 watts/per Tympani.
2- If you don't (or even if you do), the GR Research/Rythmik OB/Dipole Sub. Each sub has a pair of 12" servo-feedback, free-air woofers in an open baffle dipole frame. Sold as a DIY kit only, comes with the real good Rythmik A370 plate amp.