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Ribbons tend to have the most REAL..she is in my room singing to me sound IMO. I have a pair of small speakers that use 26 inch ribbon midrange, I've yet to hear a cone that can compete, I think ribbons have a added depth that gives notes a more rounded picture in the soundstage. This depth along with the speed with which the ribbons bring forth all wind/brass/horn/stringed types of insturments is always an amazement that I never tire of.
The ultimate midrange driver from 120 Hz to 3 kHz might well be a good pair of original Quads (the "57's"). Add a big dipole woofer or two for the bottom end and maybe a nice ribbon for the high treble, and you're pretty much set.
The requirement to go down to 120 Hz eliminates many otherwise excellent drivers. The big 3" ATC dome midrange is superb, but will need to be crossed over around 400 Hz or higher. The Pioneer 4001 compression driver is stunning, but now the minimum crossover point is up around 650 Hz (unless you use a very large horn). The 120 Hz requirement means we're really looking at midwoofers, not midranges.
The Seas Excels seem to, ah, excel in this range - but if you're building your own, roll 'em off fast on top. Some of the Bohlender-Grabener (sp?) planar magnetic drivers may also work quite well here; I'm not sure how low they can go. Audax's carbon fiber cone midwoofers are very nice as well. If it was my money we're spending, I'd probably go with Etons. Note that these drivers will probably deliver their best midrange performance either on an open baffle or in a transmission line (yes, even the planars - though the line geometry is a bit unorthodox).
Best of luck with your project!