I've had a D2 with Bose and now have a D3 with B&O, there is no comparison. The D3 Bose may have improved from the D2 version but there's a reason the B&O system was a £4K+ option.
I was always happy with the Bose system during the 9 years I owned the car, but the B&O in the D3 is in a class of it's own. The clarity and quality is better than anything I've heard in a car before, even friends who have installed aftermarket systems have commented that it's excellent.
It wouldn't be a deal clincher if I loved the prospective new car but I'd always prefer it
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Audi D3 S8 - 2007 - V10. B&O, Phantom Black, black and silver interior, and still completely standard with everything working as it should
Audi TT - 2015 - Ultra