Hello, guf.
I'm not sure this will help you or not, but I've got a Rythmik F12G myself, and I have thought about getting 2 double 8" Rythmik subs! Then, I'd place the 12" deep into the room to fill out voids/nulls. Just to be fair though, I've also considered getting 2 GR Research double 12 "H" OB subs. Someday...