"LTD02, I was planning on dual captivator imitators, but ran into a financial money pit with my home. Two major repair bills, A/C and water main replacement. I now have to fund the project selling my current subs. I didn't want to wait much longer since I have caught the bug. Idon't know if the HT-18s are good enough to make the Cap imitators."
sorry to hear about that.
the driver in the captivator is very nice. it has a huge motor and very high excursion. in powered form, it also comes with one heck of an amp. i very much like the design and engineering choices made with respect to it but for the smallish port.
the cap and the martysub are tuned quite similarly, so extension will be about the same. the si ht 18" driver is 22mm xmax, which is less than the ~30mm xmax or so in the capitivator, so one martysub will equal about 75% of a capitivator, roughly. the martysub is much larger, so doesn't require as much power and that makes it less expensive. the si driver is on sale for $219 delivered which is a really great price. two martysubs with an inuke3000dsp, dual 4 ohm drivers wired in parallel for 2 ohms each, and some handy panels from home depot...and your off to the races with huge bass on the supercheap!
it is not the end all be all of everything, but for a guy in your situation, i would not hesitate to recommend it all day long.