Some really good sealed units have been mentioned already. The JLs are nice if your budget allows. I have a pair of the HSU ULS-15s in a large, open floorplan. They offer plenty of low end in my hard to tame room. I am employing the use of an SVS AS-EQ1 to take care of some peaks and valleys. I went to the HSUs from a pair of SVS SB12s. While the SB12s didn't have the headroom I needed, they did sound great and integrated well with my mains. Their new SB13 looks nice and should get you the Quick punch. I'm just not sure it would get you into the 20s.
