The thing is with sound deadning is you dont actually need to apply a lot of the material. Ive never used Damplifier pro before so cant comment on what its like. I always use dynamat and suspect its probably exactly the same. I suggest Taking the door speaker out and placing a small square of it behind the speaker, on the actual outside part of the door. That should be sufficient for that panel, then place another couple of small squares in areas under the door card. In my experince you dont need to use huge amounts of this sort of stuff to get results well not unless your running some huge SPL system.