The cheap solution here: stop bothering with worrying about <20hz output from sealed. Just improve what is there.
Buy four Dayton 18's, apply to already paid for sealed enclosures and set all of them to nearfield. Maybe two per side, all four behind couch. Whatever works. This allows our boy Archaea to use what he bought already, get some real nice 18's that will surely up the intensity in his theater by a large factor. Easy single amp hookup/purchase. At least at first.
Want even more intensity for the cheap? Also build a riser for your main seats. If you don't have one already, I mean. Sorry, I still don't know a whole lot about your theater.
That being said, something needs to be done here. I've been following your posts now, Archaea, for at least a couple years now. I know you love your Cap's (they are nice) but you have always wondered what is on the other side of the fence. I'm not going to break it down and honestly I feel like you don't need to keep chasing after lower extension because every time you prefer the Caps. Doesn't bother me at all but let's stop beating around the bush. You ARE interested. Either go for it or maybe think about buying one or two more Caps. You obviously like them a lot and you OBVIOUSLY have the desire for more. We all get infected.
But again.... let's do this already.
Soo... what do you think of using these awesome cabs up close? Dump some decent power on them and I think you'd be happy for another long while.