Actually I built a box for the 18" Maelstrom (was about $350), added a Behringer EP2500 ($250). I think for the money you would be hard pressed to find a better value.
ThomasW lives here in Denver. I did have a chance once to experience the 12 Shivas Dancing Infinite Baffle sub. Quite an adventure to SEE the walls of his house move when impacted with 8 Hz energy.
My own system is far more modest. I own a pair of VMPS Larger Subwoofers.
An active 15" and an active 12" with a passive 15" in each cabinet. 92 db 1w/1meter. I use a QSC DCA-3422 power amp with digital EQ in bridged mono that can produce 3.4KW to hit 14 Hz flat.
Haha love the last two posts. Unfortunatly need to keep the budget under control on this one for now. I did end up buying a Cerwin Vega CLS-12s from http://www.a1components.com/ Free shipping and $300 price tag! Can't beat that! I'll give you guys an update when it gets here. I'm stoked, esp after visiting best buys and two local "home theater shops" and being disapointed with the $1000+ subs.