It would be more sealing the doors to make them into a box rather than sound deadening them surely? Either way it will improve the low freq response a little but you are still playing with speakers designed to give a full range, not solely for the lows a sub provides... So if its strong low frequencies you are after you would want a seperate sub(s)Properly sound deadening the doors will give a good increase in bass from the speakers. Doors are very poor speaker boxes (I don't think any of the major HiFi manufacturers use car doors for home systems) so the more you can make the door like a speaker box, the more you will get out of the speakers.
@Cartunes Used to recommend a footwell fitted sub, for applications where space was an issue
You could have one behind the middle seat if it folds. Or maybe a false floor in the boot if you can work a bit with wood?
The solution to this problem, as to most automotive dilemmas is to get a bigger turbo
any room for something like this?
http://s540.photobucket.com/user/iamthealchemist/media/DSC05929.jpg.html
Custom built fibreglass box using the recesses of the boot corner or side? That's what I'm having fone with a nice wee diamond ten inch sub
Photos upon completion! Some of us still prefer dB to hp!