No speakers fit as a direct replacement in an Audi unless you can get custom fit specifically for an A6 (Pioneer used to do them for A4).
You'll need adapter rings to fit any aftermarket components as the Audi fitting is non standard and the 3 fixing posts stand proud of the hole in the door card, thus leaving a 6-9mm gap all the way round the speaker. You can make your own from 9mm MDF fairly easily if you have a jigsaw and a little patience, don't think you can buy plastic ones for an A6, but they're usually pretty crappy anyway and don't fit properly.
Just cut a hole in the MDF the same size as the OEM speaker hole, then cut round the hole leaving a 2cm margin to create a ring. Using the original speaker mark the 3 holes and drill out using approx 6mm drill so that the ring fits over the mounting posts on the door card, ensuring it fits flush to the front of the speaker hole (use some silicone to get a really air tight fit). Then countersink the holes so that the original screws are flush. Then you can mount the new speaker onto the MDF.
The tweeters just slot in behind the retaining bar.
