What makes a speaker sound great at low volume?

That is one of the tests I do on a resolving system is test for low volume.  I find very efficient speakers with class wattage (zero feedback) tend to do the best.

The Fletcher Munson curve is a reality that cannot be avoided, and although efficient speakers (especially horns) do "seem" to sound better at low volume (I use Klipsch Heresy IIIs at 99db efficiency with a 12 watt per side tube amp) they can’t run from old Fletchmun...if you can live with single ended ins and outs from your preamp to amp, get a Schiit Loki...an exceptionally low noise inexpensive 4 band EQ that seems made for this sort of thing...I don’t use mine that often, but since it doesn’t drag on the signal (utterly transparent when "out of the loop") it’s always there when needed, generally to add sparkle to dull sounding recordings or kick Fletcher Munson in the gonzagas.

I find Fletcher Munson is too base heavy for my ears.  Perhaps the equal loudness contours solve that problem.   Perhaps the 12 band jriver media center eq would be useful to tweek the contour settings for my specific sensitivities. Haven’t tried it yet.  Not certain how well one would overlay the other. 

I agree with what some others have said regarding the Quads and Maggies. My first love of speakers was my Quad ESL63s. Even at low volumes, they captured the width, depth and detail of a performance, without leaving your head numb. After many years of happy listening, I had to give them up when moving into a home with a somewhat smaller listening room.
After moving back to a larger space, I considered getting another pair of Quads, but decided to give the Maggies a try. I’m now quite enjoying the little Maggie 1.7s. paired with my ADS sub. I listen mostly early mornings and late evenings at very moderate to low volumes and (like the Quads) the stage remains large and detailed, even at low volumes. Even though they’re not what you would call real efficient, they do very well, driven by my Rogue, Cronus II Mag - 100w per side, int. tube amp.
If you have the space and the budget - definitely would fit the bill, especially if you listen to jazz, light pop, bluegrass or anything with real instruments and good vocals....Jim