I can see your thinkin' but you don't have 7's? I can tell you that an $800 or maybe $600 rsw 15 will put two rw-12d's(maybe $700) to shame in real quality of sound, of course ymmv?
Cant speak for the rsw-15... but $800 can buy you alot of sub from eD, Rythmik, and Epik.