I saw a big difference in imaging when A/Bing the 803 D to S. The diamond tweeter and the extra bass driver do a lot to expand on the dynamic platform of the 803S. Sweeter mids and highs, more impactful and defined bottom-end. Better depth and soundstage coupled to a wonderful ease of listening. I've been living with the 803S on MC501's and loving them save for some sibilance occasionally, the D tweeter puts that to bed however and I am excited to own them soon! At the bottom of the evaluation list, I also appreciate the asthetics of the non-plinth look.
This comparison wad done first on an MC402 and then on a pair of MC501's (the most illustrative of this). I have 803S and am trading for the 803D. I was trully impressed and enough so that having planned to upgrade to 802D, I have switched to the 803D. It really is the sweetspot for value in the 800 line-up I think (assuming room size requirements are met and output capabilities met).
These speakers are at the very least lovers of power. They sounded great with Mcintosh, Krell (my favorite second only to the Mac as I find Krell a little too resolving for my tastes, although the D tweeter makes it so much more fun to listen to Krell for me). I didn't like them with Rotel or NAD or Bryston. Heard them with Levinson (very nice but lacking something...). They were very nice with Classe (smooooth....but less dynamic involvement for me than the Krell or Mac gear. And finally with Ayre acoustics which was great. WOnderful musicality and timing. Like the Classe but more life-like. However, not as much oomph as the Krell or Mac and way more expensive.