One potential design that I'm curious about would be a dedicated top box (intended to always be used with subs) designed for maximum sensitivity and high max SPL above 90-ish Hz. In terms of hoffmans iron law, it would be trading extension for sensitivity. It also occurs to me that woofers optimised for operation above 90Hz may also have very good performance in the upper end of their passband. I've really no idea how much sensitivity gain can be achieved by trading off the lower frequencies, but as 3db's is equivalent to a doubling of power, it seems to me that even an improvement of a couple of db's would be a worthwhile gain, assuming displacement limited and thermal power handling within the intended passband were not negatively effected. A 90Hz-100Hz xover probably seems on the high side to the HT guys, but in my experience it's fine for P.A. purposes. In fact I suspect that may HT guys are running 70-80Hz electrical xovers to keep the acoustic xover around 90-100Hz. Whether there would be enough improvement with the proposed design over say, the tempest or sentinel to justify another design remains to be seen. And before you cry "these designs are intended for home use", please remember that the aforementioned designs use P.A. drivers, and are not at all dissimilar to the typical 12"/15" woofer and horn cabs that predominate the P.A market, aside from being optimised for high fidelity and consistent directivity. I'd appreciate any constructive thoughts on what might be achievable by trading extension for sensitivity.