"Basically, MTM can indeed be centers.. just not optimal..."
whenever you have two sources playing the same content, if you are the same distance to each source, the wavelengths will be "in phase". the sound will combine. all will be good.
however, there will be points in space where the distance to the listener will be such that one source will be 1/2 wavelength greater than the distance to the other source. when this occurs, the wavelengths will be "out of phase". the sound will cancel. you will hear little or nothing. all will no longer be good.
the problem gets worse, the higher the frequencies involved (because they are shorter and shorter in distance) which means even small differences in distance to each source can create cancellations.
the problem gets worse, the further apart the sources are (because that too increases the difference in distance from one source to another unless you are sitting dead on center).
the noesis is crossed over very low, which is enabled by its expensive c.d. as a result, the wavelengths are large relative to the difference in distance from the left driver to the right driver. so unless you are almost 90 degrees sideways from the speaker, you get no meaningful cancellations.
the 8" mtm uses a coaxial driver in the center which also pushes the crossover point (from the coaxial center to the outer drivers) down to the point where the wave is large. so again, the wavelengths are large relative to the difference in distance from the left driver to the right driver. so unless you are almost 90 degrees sideways from the speaker, you get no meaningful cancellations.
here is a picture of two sources radiating waves.
the red dots show the angles where the waves will add constructively. the blue dots show where one wave will be 180 out of phase with the other and cause a cancellation.
the pattern of where those red and blue dots occur is, again, based on the frequencies the drivers are producing and the spacing in between the drivers.
for something like an mtm with the seos12 in the middle and a crossover point of about 1khz, the first set of cancellations occurs at about 30 degrees off axis. inside that window and you are pretty much fine.
outside that window and you have to consider what is going on in the room. in a very well treated room with minimal reflections, some folks will be able to pick out the point of cancellation. in most real rooms, there is sound bouncing off everything creating a real mess of a frequency response with peaks and dips everywhere. if that is the case, having such a cancellation could be good, bad, or neutral depending on all the other junk.
the original concept for the mtm was design by a guy called d'appolito. the concept was to use destructive interference from the two drivers in a horizontal arrangement to minimize reflections off the walls around the crossover frequency.
and what happens if you don't do it right:
we will hope that whoever puts together the seos mtm center does it right. :-)