If you just have speakers in the door, some damping mat will fix that just fine. The door panels are very resonant, even with just good-sounding but not overpowered speakers. Heck even the OEM system would sound much better with some damping mat.
As for the others, including the original poster, the size of their sound systems in sheer volume at the expense of sound quality, in its obvious conspicuous consumption just has to be in compensation for being small in some other way, either between the ears and/or between the legs.
There is no other way to explain such self-abusive behavior, killing your ears with the volume the massive expenditure for no useful purpose other than braggin' rights.