It can be done pretty easily and you can do it without changing the head
unit. I have heard from several experts who have worked for a/d/s and Harmon
Kardon, the the stock units in BMW and Porsche are of very good quality and
with an upgrade to the existing speakers, you will be fine. Replace the
stock speakers with aftermarket speakers, and simply make sure you are
dealing with poly speakers and rubber surrounds. Those are pretty typical
with middle of the road equipment like Alpine, Boston Acoustics and
Infinity. A few things to keep in mind...if your 993 is a component set up
you will probably want to buy a component set complete with crossovers, if
it is just a 2-way 4 ohm speaker, simply replace.
Now...sound quality...the Germans seem to be in love with 4X6 speakers as
well as 51/4 speakers. Replacing stock with aftermarket will improve your
sound somewhat, but where you are definitely suffering is on the bottom end.
You will have to add a sub, and no, it does not have to be able to rattle
teeth. A simple 8" setup, at around 100w will be more than enough. You can
do a complete sub and amp setup for under $150.00. It will make a world of
difference. If you dont want to go that route, a self contained sub works
just fine. In my S2 and currently in my 325is I simply tapped in a 50w 8"
Bazooka Sub ($89 at Circuit City.com) to the existing system and now, it
sound 100% better.
If your current speakers are not blown...add the sub first and see how you
like it.