I was thinking up firing, just mount the subs on the underside. A double layer works. IIRC, the 18" Kickers I had were double front 3/4" and had plenty of excursion.
You could down fire them, too. I'm just not a fan of that style of mounting personally. Issues with down firing is you are making a large port out of the floor and unless you design it right, you could have tuning issues. I like to have my subs play into the open space the way god intended.
