To be quite honest, regular bookshelf or floorstanding speakers are ideal for the center channel. The horizontally oriented MTM design of center channels provides awful horizontal off axis response.
The reason for using an MTM design is threefold. One, it can easily be placed flat and take up less space below a TV, two, horizontally oriented MTM designs have excellent vertical dispersion, many people end up placing the center channel above or below the TV, which is generally not at ear level, and three, a regular bookshelf laid sideways would have poor response everywhere but dead center. A vertically oriented bookshelf aimed at the MLP will give the best response and dispersion throughout the room, its just too inconvenient for most people.
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