The Audi A6 is an enviably pretty, luxurious, and capable sedan stuck in an unenviable position. Not only does it face off against two of the toughest premium sedans ever to hustle down the Autobahn--the BMW 5-Series and Mercedes-Benz E-Class--it also faces internal foes, in the form of the superlovely Audi A7 hatchback that's...