You do not have to use a speaker marketed as a "center channel speaker" for your center. Any speaker can function as a center. It is not uncommon to use the same 3 identical speakers across the front of a home theatre system. That way you know that the speakers are timbre matched across the fronts. People tend not to use bookshelf speakers in the center simply because they are tall and prefer a speaker that is longer along the bottom so that it does not block the TV. If you have the space, another Opticon 2 Mk2 would work fine, but not all manufacturers sell speakers as singles (only in pairs).