I listen at 55db-60db every morning before someone else wakes. I don't know if it is the speaker or the amp which permits a decent sound at lower volumes, but I don't feel any aspect of the music is missing other than the volume, with either of my speakers- semi-horns and Magneplaner's- they both seem to have transparency and detail at both ends of the frequency spectrum, dynamics, soundstage, just less of it, and of course a less-level response taking into account the Fletcher-Munson curve. I can't compare what my Maggies or Hornings sound like at 55db to anything else though.
Your Martin Logan's don't work well at 55db?
Regarding planar magnetics and ribbon tweeters, physics would imply less massive drivers require less power to emit the same waves, or level of volume across the frequency range, even if they are low impedance. I don't know how else to word this; hopefully people can follow what I am implying: What properties of a speaker that enables it to sound better at lower volumes. I would think this is a different concept than speaker efficiency at "normal" volumes, and they are not positively correlated? Might be something @erik_squires may have experience with?
Have you heard Magnepan's? Maybe demo some LRS+?
Maggies have always paired well with Bryston's, and you already have the subs if you feel you need them.