Recommendations for speakers that sound great at lower volume levels.

High efficiency speakers, which also have a flat and higher impedance. A number of these will be a wide band single driver design. The tricky part is that there are very very few smaller size high efficiency/high flat impedance speakers made. To achieve efficiency, physics demands some size. However with some careful consideration, certain tradeoffs allow for smaller size AND high efficiency. The main trade off will be volume and scale potential. Small high efficient speakers won't do big and loud well but will beat all others at sounding good softly.

I see no mention of Omega Speaker Systems. I recommend taking a look and a listen. Another very unique brand pretty much unknown in the US is Contrast from Ukraine. They made high efficiency high impedance crossoverless small speakers mainly using a driver of their own design and production. Those are more expensive than Omega and a bit harder to find but are exactly the sort of thing you're after.

Just to add my two cents, some of it repeating what was said above, I think a sub can help with low level listening, it adds body and little umph to make things pop like they should.

I have also found that treating the room with a mixture of absorption and diffusion can improve listening at all sound levels, but especially at low levels.  I thought my Pranafidelity monitors were much better at high levels until I added some ceiling diffusion (in addition to other treatments) and now I listen regularly at 4 - 5 dB lower levels on a routine basis.

Finally, the best low-level speakers I have experience with are the Audio Physic Steps SLEs (not the newer models which are very different) and the Totem Element Fires.  Both have full-range drivers (no cross-over) with a supplemental tweeter.  The Totems have much better bass and are easier to set-up than the Steps but the Steps, properly set-up, image like no other (at least from what I've heard).

C’mon, nothing sounds better than a Martin Logan Summit, Ethos, Spire at low volume levels. Crystal clear mids and highs, respectable bass. What more do you want? 26 tube watts will drive all of these speakers to satisfactory levels, especially at low listening levels. There is no better buy, especially on the used market.  $3k for used Ethos? You can’t top that musical excellence in a box speaker. Fogetaboutit!

Props to OP for a great topic that matters a lot in some systems but isn’t often talked about.

1st off, I have to agree with @yogiboy -- I get the best sound IMS with true acoustic suspension speakers in my nearfield/desktop system which crosses over to a good sub @70 Hz. Of the 2 sets that rock my world & sound very good at low volumes, one is a vintage unicorn while the other is a current model:

-- ATC SCM12 Pro passive monitors: the ATCs sound essentially the same at low volume as a higher volumes. That same very present, dynamic midrange and good bass are distinctly audible even with very low volume classical music

-- KEF 103.2 passive 2-way: these big vintage speakers sound very good (nuanced, detailed but not at all bright, with real punch in the mid-bass) at any volume. They’re not designed for nearfield but excel in that application. They rock like crazy at higher volumes, but even at low volumes, it’s all there.

PS: Both pairs are low efficiency (~84 dB/1 meter) & sound best with lots of power--so there goes the theory about high efficiency speakers sounding good at low volumes.

here is another +1 for ESS rebuilt Quad ESL 57's; at low volume evening listening levels they bathe you in sound...... (and Townshend super tweeters with 57's open up spatial presentation and a little more of everything)