I always matching my center channel to my mains. It helped with imaging but more important tonality and overall performance particularly in scenes when there is more than just voices coming over the center. (Opinion) having a center that melds in with your mains tonally and otherwise helps with the overall satisfaction for the HT experience; you'll hear a difference I believe when you have scenes with music and other background effects panning from L->R or R->L (or Front to back), etc...in the overall soundstage. Have you tried to find your desired center here on Audiogon? If that is still a bit pricey, could you consider the next model down in the line? There is another side to think of as well...do you like the tonality, imaging and dispersion that the center channel which matches the Summits gives? If not, that may well be the best argument for considering another center channel. I'd suggest a Legacy Silver Screen HD (excellent, have used this personally) or the equivalent B&W speaker (have heard these personally).
the conventional wisdom, with which i fully concur, is to match your center to your fronts as closely as possible so as to preserve a seamless presentation. ideally, you'd use a another ml summit as a center, or at least another ml with substantially similar drivers, impedance, etc. rears, which generally aren't as prominent in the surround mix, are less critical, so you should be fine with your vsteens.
hate to say it but what the heck.... blew my center channel tweeter awhile back and while waiting on a new unit I slapped in a bookshelf from my other system and interestingly found that the difference was much less than expected and once the movie started I never noticed it. I'd heard that you really can tell when the sound sweeps across the front but I found that very few of my movies have this sound effect. that aside yes I would match them but personally i'd not panic about it!
If the levels between fronts and center are matched well, I'd guess that Paulsax's experience would be common. Hook up one of your Vandy's in the center as a test.
If at all possible, try to use the same brand and model speaker all the way around. The resulting soundfield will be much more seamless and cohesive. I originally strted out with Gallo Ref. 3.1s as my mains and used Gallo Dues for the Center and Rear channels. This sounded pretty darn good.
But my system *really* improved when I replaced the Dues with the Gallo Reference AVs - WOW!! I now have a system that is truly good-sounding and I can now sit virtually anywhere in the room and still hear a very satisfactory soundfield.
I had to wait almost a year to find the right speakers at the right price (and I found ALL of my speakers right here on Audiogon!!), but I am SO glad that I did. I don't expect to change out my speakers for at least the next 10-15 years. And, trust me, my ears are not suffering one iota!!
I agree with the prevailing wisdom so far that matching your center to your mains is ideal. Having said that, my center does not match my mains, and it has not bothered me at all. I think part of the reason for that is that, as a generalization, the program material is different in the center than it is in the mains. Yes, sound effects that pan across all three channels will have greater discontinuity if you use a non-matching center. But what are those effects? Car drive by's and the like. How important is that to you? For me, not very.
Id be curious to learn how many guys & gals that dont have separate 2 channel and home theater systems change the center speaker when they change the front speakers. My center speaker is a Monitor Audio Silver, and on my journey to TAS, Ive paired that with Monitor Audio 10s, ProAcs, Acoustic Zen Adagios, Usher BE 718s, and now Focal BE Micro Utopias. With my Theta Casablanca II, Ive never had a problem matching the new fronts to the old center.
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.
Concure - MATCH or simply stay 2 channel up front. You wouldn't do a mismatched left and right speaker, so don't mismatch the speakers from left to center to right, either!
Quality over quantity as a default.
Wait till you find or can afford the center, or do a more modest "matched" system for the HT dubties - and leave the 2 left and rights for 2 channel only. Otherwise, stay with the stereo pair.