CS, QS8s in back certainly wouldn't be "overkill", but the question is exactly what you said, i.e. would you "want" that type speaker in back. If you haven't experienced both direct and diffusing speakers in back, you don't know which you prefer. There's a difference of opinion on this with maybe a slight edge to the more direct speakers, such as M2s.

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Enjoy the music, not the equipment.