What do you think are the downsides?
The apparent preference for wide-and-uniform pattern speakers in controlled blind listening indicates that a fairly well-energized and spectrally-correct reflection field is generally desirable from a sound quality standpoint.
Very often "kill the room" involves aggressive use of absorption, and absorption tends to be more effective at short wavelengths than at longer ones. So unless the absorption is truly broadband, what can happen is that the spectral content of the reflections is drastically altered. The ear/brain system looks at the spectral content (specifically the overtone patterns) to correctly identify reflections as such. This can be the result: At some point before it decays into inaudibility, the reflections lose so much of their overtone content that they are no longer identifiable as "signal" and become "noise".
Imo better to start out with sufficiently narrow-pattern speakers to begin with, rather than relying on absorption that significantly alters the spectral content of the reflections.
My approach is highly directional low distortion speakers, kill the room, use the BACCH cross talk cancellation SP.
Sweeet!! Do you have Janszens?
I am not certain that what works best with the BACCH system also works best for conventional two-channel. My understanding is that some in-room reflection energy is okay with the BACCH system as long as it is quite late-arriving and/or de-correlated (this based on e-mail exchanges with Dr. Choueri).
Let me ask you this, if you don't mind, since you have real-world experience with the BACCH that I lack:
If you were looking specifically for a loudspeaker with a radiation pattern that was
optimum for the BACCH, what would you look for?
Depending on the room layout and size, you can place them closer to the walls by angling... as soon as you angle them inwards towards the MLP, the back wave, then bounces off the back wall, then the side wall, before coming back into the center of the room ... which allows for a 1m /3ft distance from back wall...
Yes! I didn't go into detail about that, but you are right.