expanding foam will not solve the problem because bass isn't just pushing on the panel all the time. Regardless of how much foam you put into the middle of the panels, the subs produce negative air pressure which pulls the panels outwards as well. All you are doing is adding something else for the sheet metal to smack into.

Dynamat works by dampening incoming vibration from other panels, and weighing down the panel itself so that it takes much more force to move it. Anything lighter is useless. Thats half the reason it takes twice as much peel n seal of equivalent because they are lighter than dynamat.