Recommendations for speakers that sound great at lower volume levels.

Devore Orangutan o/96 powered with 8 watts tubes are perfect for very low level listening. I tend to agree that high efficiency speakers is the way to reach your goal. 

As a general rule, a high efficiency speaker. Over 95DB. 

Interesting idea, buddyboy1!  At a couple of hundred dollars it's definitely worth a try.

On the other hand, higher efficiency speakers are probably the way to go at the end of the day. Yes, I wish I could afford the Devore's. What say you audiogoners about the Klipsch Cornwall IV? Not cheap, but half the price of the Devores and they're rated at 102 db.

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).