For what I saw all over forums, is that the car sounds crappy whatever money you put in, there is no 'the way' of doing it, but many suggest just getting rid of the stock amp+speaker and replacing it all with aftermarket..
If you want to order a Porsche 6chan amp, be sure to get one that is NOT DSP, or you will need an extra DSP HI-FI module to get it work..
Maybe that 6chan amp with new speakers all around would do a good job for you
Good Luck