Other than trying each center speaker with your existing speakers, I would say try to find a center with the largest woofers. In order words using a Ascend with dual 6.5 inch woofers instead of a center with 5 inch woofers. I'm not saying that this approach will give you the best match, but it might be close.
I have some 37 year old AR-4x's used for left and right front speakers. Hoping to upgrade to an Ascend for the very reason I mentioned above. Now my AR's are acoustic suspension models and the Ascend is ported. Don't know how much that will affect matching the speakers.
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