It depends which '8 you are talking about - and this really will put the cat amongst the pigeons, and is
only my opinion. If you're comparing a D2 with any 6, then they're chalk and cheese - the former was pretty much a handbuilt machine, that was a groundbreaking car for Audi, and feels special for those reasons, in a way that a 6 could not do.
It created so many firsts for Audi - e.g. the first to really compete with Mercedes in this class, its first
mainstream use of a V8 by them, the first use of aluminium/space frame, etc, and it was built to a standard that reflected that.
No A6 could/can ever hold a candle to the D2, and - in my view - subsequent '8s have diluted that with every subsequent generation, as Audi sought to build volumes from '8 sales - which they have - and make money out of this "new" sub-brand, but to (some) detriment of its quality. Which is why I, personally, can only see the D2 variant of the '8 as a future classic. But that's just my opinion
