Hi John,
I would certainly agree with you that the degree of sonic improvement of using two centers is very dependent on the room setup, seating locations and the vagaries of the room itself.
But my proposal of using two centers wasn't to deliver sonic improvement; rather, it was to remove the extremely off-putting sensation that may occur with some large-screen front projection systems where the dialog gets localized at the single center channel speaker because it's too far away from the visual screen center. That is what using two centers (above and below) is meant to correct, and it does that quite well in the installations I've heard with no audible downsides.
You also said:
"The main problem with dual identical centre speakers is the havoc they play with dialogue because of the cancellations and reinforcement at various frequencies right across the room not just on axis."
I have not noted any of these effects in several installations nor in my own experiments with single and dual centers, and I would point out that "cancellations and reinforcements" are always occuring in any multiple speaker installation in any given room. Despite these effects, multi-channel music reproduction and home theater can be enormously satisfying and highly realistic.
You also noted:
"Audibility of comb filtering depends on several factors including frequency, delay, phase, reflection etc. While in some rooms the two speakers may act to anchor sound in the middle of the screen, in others there could be directivity to one or the other speaker because of the comb filtering. Duals will not always centre the channel in the middle of the screen because of these artifacts."
So far, I have not heard any installations where the use of dual centers resulted in any degradation of center channel sound or intelligibility.
Regards,
