Oh, it has an amp already? Wow, didn't know that!
In that case I presume you'll need some adapters, because I think that the amp will have only speaker-level inputs. I may be wrong, though.

I noticed a really huge difference on my dad's cars, he once had an Audi with the factory BOSE sound system, and it was pretty decent (for a factory install); his car now hasn't that goodie and, well, you clearly hear the difference. OTOH, the factory speakers on my Lupo were pretty good; as long as I kept my Blaupunkt they were enough. So you may have luck!

I see you have a radio module too; as I live in europe too, how is the reception with the module?