How about running a single MK21 as a center channel and compliment to a near-coincident MK41 pair? I haven't used my MK21s all that much, but I can say that I have not really cared for them except when run onstage. Any further back and I'd simply opt for cards or hypers, so the thought of beefing up the 41s when run up-close with a center channel MK21 might be an interesting concept to experiment with.