Best speaker design for low to mid volume?

generally a small speaker plays at lower volumes better than a larger speaker.
Not always true.. but in a broad catagory way..
I like my big Magnepan 3.6 playing at low volumes..
The most perfect low volume mid sized speaker is an electrostatic IMO. Especially Quads, or martin Logan.

IMHO, every designer takes his chosen technological elements and then finetunes them/voices them to make them as good as he can given the technology he uses and the budget he has set for production. There are great horns, great 3-cone, 3-way, 2-way, panel. Also depends on your budget. $500, $1500, $3500, $5-10K, 20K, 50K, etc...that also makes an enormous difference.

Because there are too many way to design speakers well...i would say the one feature i do find in common with nearly all great speakers...is quality of the enclosure. Very solid, no vibration, sometimes inert materials (resin-impregnated woods), aluminum frames...these materials usually speak to someone who spent time designing, is trying to create an inert box to be able then more consistently deliver sound through the cones, and create a more transparent sound. good luck. hope that was [a little] helpful...ask away with further questions.

In general I find that more efficient speakers are also better at low volume than speakers of lower efficiency. A minimal crossover might be the reason for that (more technically able A'goners correct me if I am wrong on that).

Also contributing to the quality of low level listening will be your amp. I will let others describe why this is and what you will want to look for in an amp. But in my experience, when I plugged in my recently acquired Butler 2250 one of the improvements I got was in terms of low volume listening.

In general, the more efficient a speaker is the better will play at lower volumes. Consequently, horns will be the best choice. Since you can sit 10 feet away from the speakers Avantgarde UNO comes to my mind though might not have a too high WAF.

Another way to get good sound at low volumes is to use a subwoofer. This is because in IMO one will not be able to get a healthy bass al low volumes unless he/she goes for very expensive speakers (often not even then). Since you are not interested in getting bookshelf's speakers you should get some smaller floor standers with a subwoofer. (As Elizabeth commented above smaller speakers do generally play better than larger ones at low levels).

Planars like Magnepan are fantastic at low volume. full range electrostatic speakers like the Quads also.

Box/dynamic designs that use rigid low mass materials (like paper) for woofer cones can be similarly very good also. Triangle is the line that comes to mind as an example.

I find the newer OHM Walsh speakers I also use can also be similarly quite good in a different way at low volume with the right amp and electronics in front of them, and these can also go as loud as needed with no signs of stress or dynamic compression as well with proper amplification, which is a big reason I prefer these over most planars or electrostats in the same price range.