I have Z-5500, and the center has sounded okay to me, but I've still looked for improvements. A zero-cost change that I think helped was to tilt the center speaker up a little, pointing over my head, and turning up the volume. A very low cost change was to substitute an old Kenwood speaker, LSK-2000, I got at the Salvation Army for around $18. A higher cost change I used on a different system, not the Z-5500, was to substitute an Axiom M2i for $128.