low volume(cubic feet) subs

I am beginning to design a system for my expedition, I have a kid, and use the cargo area a lot. I need to maximize my use of space for sub(s). Having been out of the car audio loop for a while I wanted to find out other opinions about subs, particularly those requiring low volume sealed enclosures. Not sure about a budget yet, I am thinking of doing 1 12" that performs well at low bass levels, and putting 2 6", or 1 8" in each front door for tight midbass punch. I used 2 6" MB quart midbass drivers, driven by a PPI a-300 in a previous vehicle and they punched hard, and produced a nice true sound. I could never get my sub quite right though. So I am going to start there this time. Any comments or recomendations would be appreciated.

check out Image Dynamics

www.imagedynamicsusa.com

most subs ARE low volume these days. Check out a selection of subs from say 10-15 years ago. 3-4 ft^3 was common for a 12-15" sub