What makes a speaker sound great at low volume?

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.

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.