Brent, unless you are hell-bent on an 18, I think it would be better in the end to run a 15 instead. In many cases, if a box is too small for the driver, it will roll off high and lack the deepest of bottom end. Would be a shame to do that to an 18. The smallest box 18 sub I have dealt with, (and I have not used many 18's so take it with a grain) is the Mach5 MJ-18. They only come in 4 or 8 SVC. They would play well in 4 cubes each sealed, but are not SPL oriented, which seems to be your goal. Maybe the best route is a pair of soild 15's that will sound great daily: Fi Q Sound Splinter RL-p SSA Icon Mach5 IXL AA Havoc etc. Or a single high output 18, such as the FI BTL, and give it plenty of space.