I run the MMC6500s in my S2000. I don't think they are 6.75" -- should be 6.5". In any event, I just had to trim the stock basket to make it essentially a ring. The only fitment "issue" is to figure out how to mount the MMC6500 tweeter. They're bigger than the stock tweeter (at least in my '05) so they could not be mounted in the same fashion as the stock tweeter.

I've never heard "MM6". Although I have never listened to the db6500s, I can tell you that the MMC6500s offer excellent mid-bass (I'm feeding them 110+ continuous watts)

 

I just put the db6500 in my S2000 last week--the mids are direct bolt in--same hole pattern as the stock speakers--didn't even need to cut the baskets. The tweeters fit in the same slot as the orginal tweeters without any modifications either--I just pried off the mounting plate from the factory tweeters (it's held on with some sort of very weak glue) and used J/B Kwik (5 minute exopy) to attach the mounting plate on the back of the new tweeters and put them right in the same place--easy install!!
They sound very good--much clearer and cleaner than the stock speakers.

www.crutchfield.com has specifications for most speakers on their website so you can check out the mounting depth, etc.

I've never heard the MOMOs or the MMCs so I can't help you in the comparision.
good luck!