Are there any direct S6 competitors in the U.S. that come with all-wheel drive?
No. However, broaden your definition of competitor and you come up with the 2007 Mercedes-Benz S550, which can be equipped with all-wheel-drive. The A6 is another story, with all-wheel-drive equipped rivals coming out of the woodwork from Acura, Infiniti, and others, but none of them offer a contender worthy of the S6.