Great Post Mark, and welcome!
It got me wondering: what would a horn enclosure do for a driver like the fostex? Will it increase the bass and leave the higher frequencies alone? My guess, from using a megaphone in my childhood (those tpys) is: a horn emphasizes a frequency range depneding on its length and cross sectional area. If that is the case, then it should be possible to design a horn for the fostex's freq curve that will emp[haisze the right ffreq range, upto about 2000 HZ to make the whole thing at least somewhat flat.
How big would such a horn be?
-akhilesh