Hi coachread and all
It may be stating the obvious but whilst you have the door panel off it would be a good idea to cover the outer door panel with dynamat or similar product. I used a special torch on material I had left over from a job its 8mm thick and has stuck well to the metal this has significantly improved the sound of the upgraded speakers I fitted but also makes the door feel really solid you know that expensive car clunk you get when closing the door. As with your findings the plastic vapour barrier on mine was totally wrecked and I used this oppurtunity to fix a 3mm thick waterproof carpet type material to act as a sound insulator/barrier.
Best regards
David Wiltshire