One thing I'm thinking about is some sound-improving insulating sheets placing in the doors behind the speakers...

Dynamat's good stuff, but don't expect it to make your stereo louder. Their claims about how it reduces the car's noise floor are accurate, but only if you completely strip the car's interior down to the bare metal and install 200 pounds of the stuff on every available surface.

I had a friend who did this to his Celica, and the thing sounded like a Cadillac inside when he was done. However, it was an immense amount of work, and that much dynamat is expensive. When he was all done, he decided it was more work/expense than it was worth.

Just putting a couple of strips of dynamat on the doors isn't really going to do much. You need to go all-out or don't bother to do it at all.

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Tony Fabris