I lived HAPPILY with the MM3 for 4 years. I am now using Magico Q1, until my Ultimate arrive end of year: cheaper, smaller, but I find them superior across the board (except of course for bass performance!). The sound is more involving, which translates into listening to more music. More transparency, more precise imaging, more emotion. I am hearing things on my favorite recording which I never heard before, with the same upstream set up: Dartzeel amp fed by APL NWO CD player. And now that I have moved to a music server feeding a PM2, a Vitus pre and the dartzeel power amp, sound is even better.It is not a question of price. The S5, objectively will stand quite well against the MM3, and will surely disappear better (I am sorry, but in your room, I don't think you can even sit far enough for the MM3 drivers to integrate properly). Subjectively, if you are happy with this kind of set up in such a small room, why look for a smaller speaker? I doubt that the S5, or the TAD will do it for you.
I have not heard the MM3 in the same room as the S5, but I would argue that the S5 will give you more texture in the bass (drivers have much much less distortion than paper drivers in MM3, you avoid technical problems created by the booster amp in the bass), but without the extension to 12Hz I had with the MM3. I am not a pipe organ fan, so not much of an issue for me.
PS: and yes, I would still take the MM3 over the Focal Stella utopia
I don't believe you can judge a very transparent speaker like the S5 (or the TAD, or even more the Q3) based on a single audition. On some sources and amps, they don't cut it at all for me. I doubt you wouldn't like them on a musical amp like Dartzeel or Vitus.