I had 12-12" JLW6's in my "winter beater" car, powered by 6 Soundstream Reference 500's... aside from the leaking back window that suddenly developed, I had several layers of Dynamat on all of the interior metal surfaces of the car - it still vibrated (go figure), but the dynamat helped a ton.
Dynamat absorbs sound waves/vibrations. Take the lid from a pan and hit it with a spoon - the lid will ring like a gong. Place your hand on the backside of it and hit it again - it won't ring, but it will "thud". That's sort of what dynamat does.