I've got a Concert head unit in my A3, running through the Bose speakers / amp. Can someone please explain the difference between the Concert, Symphony and RNS-E units? Is the Symphony any better, other than having 6-CD facility? Is it a straight swap or do I need an adapter and so on? Finally, is the RNS-E any better other than sat nav - and, again, is this easy to swap? Thanks.