Most musical instrument and PA woofers have a corrugated surround rather than a rubber or foam roll surround.
I've tried a pair of Eminence Omega Pro 18" in a car; at least a small station wagon. With about 200 watts, it achieved 140.1 according to dB Dragracing measurements. Subjectively, it didn't feel all that loud. Ported cabinet with two 6" ports (less than ideal), about 14 cubic feet.
I believe Marathon sold the same drivers with different dust caps. Or at least they were build by Eminence using very similar components. I may have a Marathon spec sheet someplace; in any case it would be educational to see what enclosure they suggested. Maybe sealed would allow a much smaller box, or the corrugated surround would allow infinite baffle mounting. The optimum volume for a single sub in a vented box was more like 8 cubic feet (for normal home/concert use).