Kyle, yes the ideal would be to have a center speaker identical to the mains, if possible. This is possble, for example, when there's enough room beneath a projection screen or a flat panel TV to use even a speaker as large as an M80 there. Where this isn't possible one alternative which in my experience works very well is to use a small vertical speaker(e.g. M2, etc.)which has similar response to the mains above the low bass, and place it immediately above or below the screen. Yet another option is to use dual center speakers either directly at the side of the screen or at the top and bottom edges.
The frequently adopted solution of course is to use an MTM configuration which as was said before was designed by Dr. D'Appolito for vertical use with a narrowed vertical dispersion, and instead for aesthetic reasons place it horizontally, contrary to the intent of the design. While this may be acceptable in general, the tests showing off-axis irregularities illustrate that it's far from optimal.
Other solutions using a horizontal shape include the TMMMT configuration of the VP150, which is said to be better in this respect. The best horizontal center I've heard used a W T/M W configuration which in essense is a small vertical speaker handling the mids and treble flanked by two woofers. At least two members here have centers from manufacturers who use that design. In the past I've wondered how an Axiom center using two of the 6 1/2" drivers flanking the regular tweeter above the 4" mid from the QS4 would do in a roughly 9" x 25" enclosure.
