Well, I've been mulling over the very helpful comments from members to my questions and have reached a decision! Having won a Symphony II+ on eBay I see this as the project for the car.
Fit Symp and connect pre-outs to a modest amp - say 4x100w - which I will locate in nearside of rear boot (assume there's no space up front?). This amp to power all speakers - looking at ditching stock spkrs in front - any recommendations for Audi suitable speakers? No sub required. Incidentally, looking at the back of the Chorus - whilst there are only two outputs for speakers I see there appear to be 4 pre-outs. Thought Chorus was 2 speaker unit as no fader option... Thanks. Dave