it takes energy to make your panels vibrate.
It gets that energy from your bass. If you prevent your panels from vibrating, your panels will absorb less energy from your bass.
Thus making your bass louder.
Of course, a cheaper/more effective, but more time consuming method is to brace your panels. Get some MDF and bolt it onto the center of your panels and bolt the mdf to the frame of the car. Your panels wont move.
Another thing you can do is use MDF like MLV (mass loaded vinyl). Basically make walls of MDF in your car. the panels won't vibrate if the sound doesn't reach them.
message all of the major names; dynamat, sounddeadenershowdown, raamat second skin, etc. for free samples. They usually give you some. test them out and see what you like