John John, Bass-low frequency- is pretty much omni-directional (a room or XJ) Ya kinda feel it as much as hear it.
24-7, are those kick panels custom or the Q-Logic pods? That 'has' to be the ideal setup for sound quality up front.
I was web-shopping the other night... mainly for front component speakers, but found this:
http://www.crutchfield.com/S-O4gwYiKJinA/cgi-bin/ProdView.asp?s=0&c=10&g=51000&I=108BASSLT&o=p&a=0&cc=01&avf=N&search=
It will fit an XJ, bolted to either the rear seatback or the lower hatch. The drawbacks are it's 'fixed' and would interfere with a full-size spare mounted either place...it wouldn't intrude on cargo room or floor space much though, esp. on the hatch.
$500 is a bunch for a complete sub setup? or is it? That one takes up very little room.