"Jkalman, the sibilance is a phase distortion issue & it won't go away with toe-in (like Zd542 wrote). I bet that whatever you try you will not be able to get rid of it. It's inherent in the speaker design & the 4th order x-over. The boomy bass is due to the poor port design. B&W cannot get this right & didn't get it right even in my DM604S2 nor in my friend's 803D. Port design is hard to start with - you have to start & stop a slug of air at a moment's notice given the fact that the bass from the port is 180 degrees out-of-phase with the bass cone driver. If you don't design this correctly, you get bass overhang or what you call boomy bass - the previous bass notes hung around too long & interfere with the next bass notes (from the next bit of music) & smear it.
Play some tracks from Diana Krall's Love Scenes - sibilance city if you will. Let us know at point in time you walked out of the room!"
My understanding is that they don't use a 4th order crossover in the 803 D3s (and haven't in their high end diamond speakers for some time). They use a first order crossover.
In any case, playing around with the room placement and toe-in some more eliminated the issues I was having with the sound. I never said there was problems in the bass frequencies. It was a bit of booming in the mid-range, mostly in the lower register of voices, which don't come close to the bass ranges. The excessive sibilance was from having the tweeters pointed in too much. Sibilance is natural in the human voice, especially in the female voice. Finding the point at which it sounds natural is the goal, not eliminating it completely.
Actually, acoustically speaking, the smearing of notes will happen more because you haver the speaker to close to a wall than anything else. When there is not enough time delay between the direct sound from the speakers and the sound from the first point reflections, then the sounds become smeared, otherwise it becomes ambience. Though, too much ambience can color the sound with the character of the room, and that isn't good either (thus, the need to properly treat first reflection points). The port on the back shouldn't be an issue unless you are already making the mistake of bad speaker placement.
The other issue is placing the speaker too far into the room, at which point locality of sound becomes too diffuse. That is problematic as well.
BTW, I bought them, and it looks like I will be the first person in the USA, or possibly the world to own a pair of the 803 D3s! :D