Phusis,
Many good points are made in your excellent posting. It would indeed be challenging for a big woofer to handle frequencies up to 700 hz. A local builder uses quite sizeable horns and compression drivers that can go extremely low. I recently heard a system where the crossover from compression midrange to woofers was set at 180 hz with a very shallow 6 db/octave slope. The compression drivers were quite pricey, and if this were my system, I would insist on a higher crossover point and/or a 12 db/octave slope. The woofers are twin 18” woofers made to his specifications. The manufacturer insists on a minimum order of 40, and in the past three years or so, he has made three such orders.