I run a generic (Best Buy Special) powered 8" tube with a JBL sub in it in the back of my Trooper. The bass is more than enough for me, the only issue is mounting it. I really like being able to fold the seats down, and the sub gets in the way. Right now I have it strapped to the driver's side forward cargo tie-down. I can release that and slide it around a bit (within the confines of the wires) to accomodate different types of cargo.
So, maybe I didn't answer your whole question, but the size isn't horrible.
Tubes would take up a lot less room than my set up (2x 10" mounted on each wheel well in custom boxes, Amps mounted on small rear door) and can be mounted in any direction. If you mounted it virtical between the rear door and pass side wheel well (assuming your small door is pass side) you should have no real loss of cargo room.