@asvjerry: For woofers that can "keep pace" with your Heil AMT’s, check out the OB/Dipole Sub offered by GR Research in collaboration with Rythmik Audio. 12" servo-feedback woofers (two or three) mounted in an H-frame (or M-frame if you prefer, in which case for two woofers only), with the Rythmik A370 plate amp into which is installed a dipole-cancellation compensation shelving circuit (which enables the sub to reproduce frequencies into the teens). That the sub shares the dipole radiation characteristics of the Heil driver makes it particularly appropriate for that application. Unlike "normal" subs, usable up to 300Hz. Details on the GR Research and Rythmik Audio websites, discussion about the sub on the AudioCircle GR Research Forum.
Ric Schultz (ricevs above) used that very sub system in the excellent loudspeakers he was offering when all the NEO Magnetic-Planar drivers (Neo3, Neo8, Neo10) were available. Known for their "lean" (none of the all-too common "plumpness" of most subs), clean ("stop-on-a-dime" characteristic), "fast" (return to rest when the signal stops, with no "overshoot") sound quality.