Good idea. FWIW, a lot of folks say that the center becomes more imperative when the viewers are spread out. However, I personally think it's important to point out that this is EXACTLY when the weakness of a horiz MTM rears its ugly head. The centering might* not be quite as good, but the dialogue intelligibility is just about always better in my experiences, when being off axis and comparing phantom vs horiz mtm. Again, just fwiw.
Yes. It's not a 3 way vs 2 way issue. The specific problem is the
redundant drivers playing the same exact audio, while on the same horiz plane. Once you are at
different distances from these two woofers, playing the same exact thing, you suffer random sets of boosts and cancellations depending on the angle you sit, due to the interference known as "lobing". What makes the typical horiz mtm so bad is what I alluded to before, and that is because the xover point is so high, that the set of lobing frequencies is very great, covers an enormous part of the freq spectrum, and inevitably compromises dialogue intelligibility.
That's the crux of the paradox. People tend to recommend the center speaker when being offaxis, yet it is precisely when you are off axis that it sounds worst, as far as the individual center speaker being mtm. If you are rather on axis to the speaker, I think mtm is fine. Depending on the design, normally you want to be within 20 deg cone, but I think straight on is the only way.
Now, ok, you're right in the sense of the 3 way, because having that
single dedicated midrange driver allows the woofers to xover much lower from the rest of the speaker, so that lobing issues are much less offensive. In fact, some WTMWs (you guessed it, B&W), are designed so well with horiz WTMW, that lobing is simply inaudible (a big reason being that lobing starts very low, where we are insensitive to the effects).
Other tidbits/factoids: a horiz MTM could be better designed if the woofers are closer together (the "differing distances" are reduced when off axis, make sense?), and the xover point is lowered. However, I don't think I've ever once seen such a design, though I'm sure they exist. I'm sure the issues with driver limitations must come into play.
I copy/pasted some results, with highlighting here:
http://forums.audioholics.com/forums/showpost.php?p=632842&postcount=11
which came directly from this AH article:
http://www.audioholics.com/education/loudspeaker-basics/vertical-vs-horizontal-speaker-designs/conclusion-rankings-and-evaluation.html
I didn't google it sorry. What I will say is that the actual best possible match for your center speaker is yet another P4. Best is to put it vertical. If above TV, it's fine to flip it upside down. If horiz, you suffer lobing at and around the xover point, but it's better in the sense it's not
everything below the xover point.