You need an amp that will take high level inputs as the standard headunuit is very unlikely to have low level outputs.
You will also probably want to run some rear speakers on the parcel shelf or in the rear cards off the amp as you will need to disconnect your rear speakers from the headunit as you will need the feed that powers the rear speakers to go into your amp instead.
I also have a Design and don't want to chnage the standard HU because it is piano black and looks well nice, will look **** if put a aftermarket HU in. When I first got the car I wanted to upgrade the ICE byt after a while I got used to it and am actualy very impressed with how good the standard ICE is in the new D.
The advice I have given is my understanding, but i'm no expert so if I'm wrong somone please shout at me!!!! :lol: