Well, given that I respect your opinion and research so much FINE we'll do it your way! wink ::stomps away like a toddler::

I think what I'll do is remove the boxes and add a layer of MDF to the back with GE Silicone II sandwiched between. This should make me happy with the ringing problem and increase mass. Then I'll get some thinner insulation to line the joist space directly above the speaker. I can get 1" thick stuff I think. I'll add whatever is left over to the inside of the cabinets on the down facing side opposite the rear panel.

I agree the knuckle tests arent telling full the story too.