I heard that subs that can go down that low (19hz) have to be servo-controlled. I don't know that much about it though.

I'm currently reading up on http://www.diysubwoofers.org/, trying to get some ideas. I'm pretty handy with a soldering iron, and would looooooove to have 19hz bass in the house... tho definitely not in the studio, I'm sure it'd throw off my monitoring setup...