Well I just measured mine and there is only 2" under it haha... BUT..the bottom of the couch has just a thin wrapping that you can press in and see that its hollow beneith. I think this is just to make the couch look finished. I didn't take a knife to it (yet) to see how much room is actually under there... but I bet there is some real room. Just pushing I get like 5 or 6".
I think this project is neatest if we designed something that only fit under most couches and didn't use anything behing. Because, lots of people have their couch sitting in the middle of the room so behind wouldn't work for hiding it. If there is room behind and its just up against a wall, then lots of stuff could be fit back there, even my giant. In that case, basically what I call a "truck box design" would work, like the car audio guys build for regular cab pickup trucks that fit the profile of the seat versus the rear firewall.
I bet under most of our couches is a lot ton of dead space. I think the design would require a low profile driver to work for most people. So far we have a Tang Bang and a Diamond, and I just found this on JL's website:
http://mobile.jlaudio.com/products_subs.php?series_id=30
3-ohm, just as a note.