I have a carrera 4 and was faced with this same problem. I went with option number 2 (connect the empeg to the external amp, and the CDR220 to the empeg's aux in) and it works great. I did not do the install myself; I managed to find a car audio shop in town (Santa Barbara, CA) that was willing to do it, much to my surprise. At first they weren't sure if it would work but they had no problems at all doing the install.

(I know this is an old thread but thought I'd reply anyway in case anyone else is doing a search for porsche info...)