I think the reason why the MX660 seems to have a larger soundstage than the MX760 is because of the recessed midrange, but that's just what I'm hearing. Of course, I'll take the MX760 over the MX660 any day.
I think my MX-660s may have been fakes. The cables at the Y split were severely crimped and the metal benders in the top of the leather bag split open after about 2 month use. Being fake might be the reason mine sounded like it has a super-wide soundstage -- almost like a concert-hall sound processor.