A Porsche forum would probably be your best bet. No doubt there are forums dedicated to the 944.
As a veteran mobile audio installer, I can tell you that you’ll definitely want a system that allows for EQ, preferably by way of DSP. I imagine some headunits have it built-in these days. You can get analog EQ solutions from the Audio Control company. I’m not sure what the top brands are these days. Years ago, they were Eclipse, Audison, Hertz, Memphis, Pioneer Elite, and Soundstream to name a few.
I owned a near-mint 89’ cobalt blue 944S when stationed in Germany 16 years ago. Fun car but abysmal to drive in the snow.