Hi,
Thanks, Joema, for the mention and link to Floyd Toole's white paper. It was Floyd who first demonstrated to me (back in the late '70s) the effects of comb filtering and the limitations inherent in 2-channel stereo playback.
Tony, ALL speakers at the front of a room produce comb filtering and uneven response as a result. It's yet a further liability of stereo listening and is quite audible using pink noise as a test signal. The explanation is too long to go into at length here but while it's audible with pink noise, it's not a significant factor that seriously undermines our enjoyment of music, movies, or even stereo playback, nor is it identifiable using music as a test signal. (Music has a wonderful masking effect of tiny and even large distortions.)
Plus I believe you had your VP150 stuck in an entertainment center, correct? I would suspect that the interference effects of the EC would have the potential to significantly alter the dispersion of the VP150 and introduce some of the anomalies you seem to find with it. A speaker with a smaller enclosure might be less affected, which might explain why you aren't finding the same effects with the smaller M2i on its side.
All of that said, I would point out that the stereo illusion and that with multichannel is always compromised to a greater or lesser degree. What is astonishing is that with neutral, transparent, and accurate speakers like Axiom (and some others) that the movie soundtrack soundfield reproduction with 5.1 or multichannel music is so convincing and enjoyable much of the time!
Given the vagaries of listening rooms, furniture and the like, it's a hopeless and frustrating odyssey trying to find the "perfect" center channel. There isn't one. But the VP100 and VP150, when properly set up, deliver a convincing and seamless soundstage and intelligible dialog for a reasonable number of listeners in most average rooms.
Regards,
