What makes a speaker sound great at low volume?

omega speakers! they are simple fantastic for low level listening. and no slouch @ 11 either (:
pair them with decware or linear tube audio and you'll need spend 10K to do better, just sayin

Its all about timing. At low sound levels the ambient room noise dominates and the electronics and speakers move off their ’sweet’ spot. But you can still sense the ’grip’ of the music and vocals if the transient timing is preserved. The three Casablanca audio clips in this paper are particularly enlightening:
https://m.phys.org/news/2013-02-human-fourier-uncertainty-principle.html#
So, i was able to supplement my system with a TakeT BatPro2 super tweeter (which is a just a phenomenally fast tweeter) which maintains the energy of the transients to make everything audible and makes low level listening very enjoyable.

Two suggestions that I have owned, and sound really good at low volumes: Monitor Audio GR10's, matched with a NAD 326BEE - modestly priced combo that sounded fantastic (still regret selling) - and Ascend Acoustics Sierra 2's (which I still own and love), paired with a Rga Elex-R.

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.