matching center channel to mains/rears

Two more considerations:

1. Room correction software can aid a bit in speaker matching, making a non identical center blend better.

2. If you do get matching pieces, they might be easier to sell as such later, but in reality the matching may hold you back from spending more elsewhere in your system now. i.e. putting the money saved from "lesser" center into better mains, cables or just movies.

The most important speakers to match are the 3 on the front. In order to be matched correctly, the drivers should have the same size and made of same materials.

I think your best sound will come from matching brands

FWIW, I had Paradigm Studio 60 v4 fronts and a CC-590 v4 center. I then sold the Studio 60's, kept the CC-590 center, and moved in Dali Helicon 400's MkI for the fronts. Based on all that I had heard and read, I was expecting to be dissapointed with the mismatched fronts/center, but I really wasn't at all. Is it ideal? Probably not but don't automatically assume just because you "upgrade" your fronts you automatically have to plan on a new center channel also. Try it first, and you may be perfectly satisfied with mismatched fronts/center. I would like to eventually get a Helicon 200 center, but I have to say, that the Dali's mate quite will with the Paradigm center.

IMHO, matching the fronts to the center is not nearly as big of an issue as it is constantly made out to be on the forums.

I would definitely stick with ML in order to match the tonality and speed of your Summits... either the Stage or Motif. I have Summits with a Motif center and couldn't be happier.