Then you should email Jim Salk about building you Rythmik 18"s in his cabinets. Otherwise, anything you buy will likely "take a hit." If you are worried about room sizes... shop for the best possible sub for covering a >5000'3 room, buy at least two, and you should be good. (That goes back to the Bassaholic room ratings here at AH.)
JTR and Deep Sea offer some nice subs. But unless you're eager to get into the Orbit Shifters, or go all in on $8K Funks... we come back to square one: Shop for the features you want or need to match your system, and know you are in reasonably good hands with these companies!