The Fostex horns you want to use need to be crossed over at 3KHz minimum (12dB/octave) and drop like a rock past 10KHz. Finding a 12" driver that has good response out to 5-6KHz is gonna be tough. The 12" drivers are going to "beam" a lot above 1 KHz or so, too (response off axis will be ragged). You might want to try something like an Eminence Beta-12A-II in ported box, it has a 1w sensitivity of 98dB and will be close to the 102dB sensitivity of Fostex units. You should be able to get fairly flat down to around 45-50 Hz in a modest 3-4 cu ft enclosure and still have good efficiency. You can get the Eminence from Parts Express for around $65 or so, and a cheapo crossover (12 dB - 3.5 or 4KHz for around $20 or so. Just my 2 cents...!