center channel speakers - Martin Logan vs Definitive Technology?

The center channel should match the L&R. If the matching center channel to your L&R is too large, you should at least use something from the same company.

Ditch the cabinet. 

I had a similar issue needing a slim center channel to fit on my mantel in the living room and went with the ML motion 8...It doesn't perfectly match my L&R but the specs lined up pretty closely and I'm happy with the results given the real life limitations I was stuck with (the room and keeping the wife happy). If I didn't know it was there, I probably wouldn't hear the slight difference in sound even during the occasional LCR panning sequences. The motion 8 is a solid speaker, but if ditching the cabinet isn't a realistic option then I recommend choosing the one that most closely matches the specs/profile of your L&R speakers. Better to get something that integrates well with your existing system than getting the one that sounds "best" on its own.

Thanks to everyone except the idiot who said "ditch the cabinet"

I did go with the Definitive Technology Mythos Seven and can say there was an immediate, noticeable difference in the the sounds - specifically voices and various dialogue. I am sure the ML would probably have had a similar result.

Sorry man.  At this site the order of operations is generally:

1) Audio/video quality
2) Value
3) Humor
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.
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256) Cabinetry

My response was definitely aimed at #2 (but I was also feeling your pain with your size limits), so very sorry if I offended and don't hate me forever.  Congrats on your new center speaker and I'm genuinely glad you're happy with it!  Hopefully it gets even better as it breaks in.  Nothing worse than being disappointed or underwhelmed by a center channel speaker.  Ruins the whole HT experience, and I've got a bunch of friends who skimped in this area to save 100 bucks or so and it's just awful.  Anyway, enjoy!!!