use small sections of 2x4 or double up on your layers of panels. ... or at best, redo the install out of 1" thick birch, that shit WONT flex, plus its water/air tight. not cheap but best wood for building cabs out of. i've seen all sorts of tricks for deadening and SPL applications... red clay in A/B pillars, sprayfoam in doors/trunks/etc. even diamondplate and concrete filler strengthening boxes. even used 2x2 box steel to keep ceilings from flexing. if it moves, it absorbs sound pressure.