Well I definitely appreciate the inclusion of my moofas in this discussion but they are gone pending a deal done on Friday. Therefore, and since he has shown no interest in them, I'd just like to ask some questions. Why are you set on a pair of 18s if you're not looking for all out SPL?
And yes I am well aware of the fact that a pair of 18s can have great SQ, such as the MJs that have been suggested. However, why the need to pull out the 3rd row seat and throw a pair of 18s in the back when only looking for a little bass (not something to kill your ears, only like 120 dB)? Also, I think you may not be completely clear on how loud certain dBs are.
But for your application, a single 12 or 15 would be perfectly fine. Or are you looking to be able to show people 2 18s in the back?
Those are my questions, just wondering why you would want to take up so much space with 2 18s off 1000 watts when I feel that there are subs that could use less space and get as loud while sounding crystal clear, with just more power.
Anyway, as far as 2 18s to fit the bill for cheap, solid, sound quality oriented woofers, I support the idea to purchase a pair of MJ-18s. They are probably about the best choice possible because they are very inexpensive for their quality.