I haven't personally heard it, but I would add Milltek to the list of candidates. Not so big in the US but they do make good quality exhaust systems. See
http://www.milltek.co.uk/media.file.cfm?fileid=566 - I had a full Milltek system (including down pipes) on my old RS4 and it only droned at 1500RPM which wasn't an issue. The catback system was quite a bit quieter, almost like stock just slightly louder.
Another option would be Akrapovic slip-on system, but reasonably pricey since it is Titanium...
Final option would be to get your stock exhaust system polished or similar to make it look nicer...