OP, forget everything I recommended about center speakers.
According to kiwi2, horizontal center speakers are the bestest best decision you could make regarding dialogue clarity. A horizontal center is even better than a 3rd identical speaker as your mains. Yes, those that recommended a 3rd identical speaker as your mains were completely wrong. In fact, OP, place your MA rx8's horizontally too. Horizontal the best, the wider the better. They will sound fantastic like that.