Take a look at this thread from last year. The guy uploaded a video of his setup so you can listen for yourself
http://www.6post.com/forums/showthre...t=exhaust+640d
He used one of these kits
http://www.active-sound.eu/de/bmw/6er-series/f-series
It's called active sound but with that sound box welded to the exhaust, it will be giving off some throaty outside noise, not just piping some nice engine noise through your speakers