Mike, I love this thread!
I have a friend who does some very big nightclub installations, and he always tells me the trick to getting the "punch" is actually in the lower midrange. The bass provides the vibrations and the warm feeling in your ears, but you need big powerful mains for that feeling of getting hit with rocks each time the bass hits. With car audio, like you spoke of, it's a lot easier to get to that point because the listening space is so small. Your mains are big, but, they aren't going to keep up with all the air your subs are moving. I think your next step should be buying some new speakers, maybe even pro-theater mains. Bag End, JBL, Klipsch...
Have you ever checked out Bag End?
http://www.bagend.com/
They have some subwoofers that could kill a small animal. They even make a Quad 18"
...
They have some very nice sounding, practical, mains as well, and if I ever could build a real theater for myself and not have to worry about neighbors, that's probably the route I would go.