What makes a speaker sound great at low volume?

High efficiency (high nominal impedance and sensitivity). Other parts of the chain can have an influence but efficiency is the primary factor.

High efficiency or electrostatics!

What makes a good speaker sound good at low volumes ?
 "" A good Speaker"" and listen to it !

From my understanding Frequency Response changes with Volume.
This is  why you see Loudness contour Switches.
i remember Yamaha amps had adjustable Contour Control.
I remember reading The Fletcher-Munson equal Loudness curves.
https://www.teachmeaudio.com/recording/sound-reproduction/fletcher-munson-curves/

I auditioned speakers on modest dollar for just this issue, with my wife who has great ears but hates higher volumes. Mostly classical jazz and acoustic. Walked into shop expecting to buy the KEF's, came home with Rega RS3. Elegant sound.  Adding an SVS sub eventually added the bass fine tuning at lower levels. And my modded Denon pre has great loudness control if needed.
A friend's Vandersteen II's with great electronics sounds fine on lower volume acoustic stuff also. But as others said if I had money and space for them I would have Quads or Maggies.  Maybe later.  Agree that placement and listener position huge. Near field likely the way to go.