It's quite possible Len you've never had a loud enough system, because regardless of how many swage lines you have on the exterior of the door it will vibrate (resonate). Plus if you remove the door panel of a MK2 and close the door, it resonates like buggery, hence the reason for fitting sound deadoning to stop vibrations.
I work with helicopters and they have a sh1te load of sound deadoning to stop panel vibration and cut down on turbine noise, so I personally would say it does work, and is worth carrying out on cars whose owners are serious music listeners.
Dynamat isn't the only product out there, it's just been recomended to me by more people than I care to name, and if you can afford it 9 times out of 10 you buy 'the best'. IMO
By the way, just in case some commision is available my paypal address is.......................................