sub enclosure mounted to chassy

I finally finished installing my system in my 2005 accord.  I am very satisfied with the looks and sound with the exception of the vibration throughout my vehicle.  I used plenty of sound deadner but still have lots of vibration resonating throughout the vehicle.  I am wondering if this has anything to do with the installation of my enclosure.  I customized the box to fit against the back seat and basically bolted the enclosure to the back seat frame.  The enclosure is extremely secure and does not move at all.  It would take someone lots of effort to remove the enclosure.  I am wondering if most of the vibration is coming from the  box which is mounted to the chassy causing the heavy rumbling throughout the vehicle.  Does anyone have any experience with free standing(box not mounted against any walls) enclosures versus enclosure mounted to the car frame.  Does this cause more vibrating effects?  Thanks.

While its recommended to brace...Remove it from the chassis and see what happens...M.E.C.P & First-Class

Go slow and drink lots of water...Procrastinators' Unite...Tomorrow!