racer69 - thank you for the pictures and the idea! There is a product called TUFF-R (and Super TUFF-R), which is similar to the pink foam boar that you used. It is also closed-cell foam board, but with slightly higher R value (R = 6.5 per 1"); it is covered with an aluminum foil on both sides. Is there any reason I should not use this instead of pink foam board? Is aluminum-foil backing going to create any problems for in-wall speakers (I have ones without an inclosure, with an open back end)?
One more thing - did you have a chance to listen to your set-up to see if it affects sound?