I stuffed some fiberfill behind my stock door subs. I think this helped to dampen a bit but it is not a huge difference. But for the cost of a cheap pillow
, every bit helps
Another cheap improvement is to use duct sealant on your 5x7 mounts. It is a clay-like material that you can press into place and cover with aluminized duct tape. It does not harden and is heat resistant. It helps with sound leaks and deadens rattles and vibrations as well, especially on the flimsy stock door brackets.
I got it at my local HW store for around $10
This is a home audio application, but same idea http://www.audiokarma.org/forums/sho...d.php?t=283961