Do you already have the SSD12? If not, FiAudio may still have some of these X-series drivers left.
http://www.audiojunkies.com/blog/712
A pair of 12" X-series drivers for $160 shipped! Those in 8ft^3, VCs in series, drivers in parallel, tuned to 20Hz with 3 4" ports and hooked up to that Oaudio amp with the SSF set to 20Hz. Yeah, that'd work.