I've got an Edition 30 with the standard head unit (I think it's an RCD 500?) which has an old-style iPhone connector in the glove box.
Using an adapter from the old iPhone connector to a lightning connector I plug in my iPhone or iPad to listen to music and I don't think the sound quality is as good as I'd expect, it sounds slightly compressed and tinny, definitely not as good as a CD. I'm using the Spotify app at extreme quality for playback and if I plug the iPhone or iPad into a portable speaker with an aux cable the quality is as good as I'd expect so I don't think the issue is with the device itself.
Is this a known issue with this older head unit? My girlfriend isn't convinced she can hear a difference but she would say that to stop me buying a new head unit!