I spent ages calling around car audio places and looking for a solution on the web. There were suggestions of 'door builds', pods, shelves, etc etc. Mine is a 1986 944S. The available space in the front door trims is very limited. In the end, I went for the best / largest that would fit in the front (still tiny) and added some dash mounted 'Morel' tweeters and the largest possible speaker replacements in the rear panels (which involved removal of the rear windows and a spendy install at the car audio place). This sounded 'clean' but had ZERO bass so I added a high quality 'Bazooka' bass tube in the boot.
http://www.sasbazooka.com/bass tube
End result is not bad. Total cost was about £600 + the head unit which I already had. If you are looking for an audio place in London, I went to a really well known one in North West London which was first class. I could dig out the receipt for the name/number if that would help?
Good luck with it. You have to do something as the standard set up is terrible, even by mid 1980s standards!!