door speaker upgrade for quads

I recently replaced the head unit and original equipment door speakers in my quad. The original speakers' cones had turned to dust, and I thought installing some new Jensens with a Sony head unit would produce a significant improvement in sound quality. Understanding that the door panel attachment for the speakers makes a poor enclosure, I still expected better results than what I got. The bass response is about non-existant. I have checked to confirm that the speakers are "in phase". Has anyone had any success in improving the sound quality from this type of set-up? Stuffing the enclosure with insulation would no doubt help, but the way the speaker cover attaches to the door panel makes that a tricky job.