www.fourtitude.com is VWVortex's Audi site. That's probably the best you're going to find. Their A6 forum is pretty slow, though.
Audiworld has good info, if you can pick past the fanboys and their crap.
Audizine is OK, too.
The trouble is that most people on these forums are interested in, for lack of a better term, "tuning". There are some mature people interested in their cars, but they're hard to come by, or so it seems.
A friend of mine has a really nice A6 2.7T. Very tastefully modded w. BBS CH wheels, eibach pro-kit, magnaflow axle-back exhaust, GIAC software, RS6 grills and seats, etc etc. He's an older guy, so he'd not trying to impress anyone with bling or blow-off valves; he just wants a nice clean ride.