B&W line up

I can help here.  I'll keep it simple and stick to the 800 series.  Essentially the smaller the number, the bigger the speaker.  805 is the smallest bookshelf speakers of the 800 series (been this way since the early 90s) then 804 is the smallest floor stander and 800/801 are the rarest and largest floor standers of the 800 series.

Series wise, starting with the 800 Matrix series in the 90s, followed by the Nautilus series, then the S series, then the D (aka. Diamond) series, then D2 (aka. Diamond 2), D3 and the latest series is called the D4.

In general, the older B&W 800 series has warm and full sound but less high resolution transparency.  Each newer generation uses better crossover components and speaker cone materials to increase the clarify and resolution but in a compromise becomes loses the warmth and fullness.  The biggest difference in the latest D3/D4 generation are the shocking price tag increase and that the even the smallest floor stander 804 D4 can generate sub bass frequency going all the way to 24hz (@-3db) and 20hz (@-6db).  This is very difficult to reproduce by other speaker manufacturers without using huge 10+" bass woofers.

@hifineubee you're saying the D4 series has that much better bass response over the D3 series?  Driver and cabinet wise, they appear pretty much identical. I'm not sure how they squeezed that much more bass out of them but kudos..

It was said above the the older 800 series were "warm and full"... I can definitely agree, however, there is a midrange crispiness that I feel is warm YET resolving at the same time. I heard the 801 D2 and they were nice but wasn't overly impressed.  My older 801 s2's made me smile though when I hooked them up. Just makes me want to listen.. I had the same experience with my Duntech Crown Princes and Martin Logan 13a's..   Just ahh nice