I agree with Spenav. What you're trying to achieve is fundamentally not possible. A true center image is only possible by one listener sitting precisely between the speakers. Set your speakers and chair up just for one person and then don't tinker with them!
Speaker set up for more than 1 person
I have my system set up perfectly for a single person sweet spot. Near field about 9 feet from my speakers. But if I move even slightly off center the soundstage moves and one of the speakers dominates. If I have a couple friends over how can I arrange my speakers so we can all get a good soundstage with centered imaging? Move my speakers closer together?