You technically want to deaden the cavity that the speaker is mounted in...however most of the time it is extremly difficult.... so what you want to do is take a strip about 2 inch wide down the center (or close to it) of the inside of the door skin and a round piece directly behind the speaker... This takes the resonance out of your doors...... then completely cover the the face of the door in dynamat leaving only holes for wiring and mounting points...This will stop wave cancellation.

I prefer dynamat extreme my self due to the fact it sticks like glue and its thin...there are other companies out there but I use what I know works....
oh as a side note in oder to prevent clipping it is all how you set up your equipment..my rule of thumb is... your HU should have bass,mid,trebel all at zero and you gain youre amps at 80% HU volume and go to each amp and dial it to where you like it(b4 clipping occurs of course)... then u can crank it there all day long no problems.... make your X-overs do all the work your HU will sing

As always just opinion