Better? Hard to say because they're completely different speakers!
Klipsch Cornwall III is a very large cabinet with a 15" woofer. It's more sensitive so needs less power than the B&W 804 D3 to get to the same SPL. If you like the Klipsch Cornwall sound, you will find the B&W 804 D3 to be perhaps "clearer" and faster-acting from mid-bass on up, but they can't touch the bass you get from the Cornwall III's. You would need a subwoofer to get there.
Overall quality of sound, I happen to prefer the B&W. You may not.