Took a peek and as I suspicioned it's huge to go low with such a high Qt and instead of an 'H' it ideally needs a flat baffle on one side with the 1/2 H 2x larger and opened up till just slightly folded to get the desired 'flat' to 200 Hz+ at high eff., so best to put both drivers on this huge monopole positioned somewhere between the mains hidden behind some weighted drapes to further damp it to 'taste'.
That said, Davor's little two stage pipe tuned a little lower seems a much more viable option.
Yeah, attic/basement IB or in one case a ~300 ft^3 false wall IB added to a concrete basement HT.