if this is meant to be powered by an avr, you may wish to consider using a woofer with less weight and power handling in order to keep efficiency/sensitivity as high as possible. kind of in the spirit of the old skool builds.
having a voice coil that can take 2000 watts and a cone/surround/spider all up to the task beat efficiency down and with 100 watts or so, the system won't be coming anywhere close to using all the voice coil excursion.
in order to get the most sensitivity out of the system, a lightweight driver, such as the old altec 416-8C is the way to go. gosh, I wonder why altec designed that driver that way? ;-)
a horn is always interesting, but even for 30hz, the length would have to be somewhere in the 12.5 feet long region. not that it can't be done, just might be a little more size/complexity than you want to bite into.
i'm not completely sure that I understand what the application is for this product. it would seem much better to have an active crossover for the lower end where all the power will be used and this is what a typical avr does with the subwoofer output. ?