It would be much better to use a few smaller subs distributed in the room to smooth room modes. So my suggestion is to take the same pair of LAB12 drivers you might have put into a LABhorn and put them in 4.5ft3 cabinets port tuned to 20Hz. Overlap them with the woofers in your mains giving you four bass sound sources, distributed throughout the room. This will give you plenty of bass energy, and it will average the energy throughout the room in the modal region giving smooth bass response.