I was in the same position and Ray recommended John Kleis who they tend to use for anything more involved than just changing a head-unit. I am thinking of putting a becker cascade into my 993 and have the sound pack too. Am tempted to try and swap the amp and door speakers if i can use the existing wiring. not interested in a big audio install as i like to keep the car as original as possible, but the factory system is rubbish.
I replaced the front door speakers in my old 993 C2s and it made a big difference, even with the standard head-unit. The Sound pack makes any changes more complicated though.
For what its worth, i think the switch of the OEM headunit to a becker may need an ISO harness adapter. I tried to plug 'n play my cascade and got no sound (think the remote out on the headunits are different).
If i remember correctly, there is a german company that makes all sorts of harness adapters (Kaufatec i think?).