What makes a speaker sound great at low volume?

No crossover. Tube amp directly driving the woofer...

+1 .. my Decware/Omega pairing continually surprises me with the level of detail and dynamics when listening at low volumes levels. 

A few people have suggested ESLs, and I'm pretty sure they'd fit the bill. I had ESL's with a McIntosh set-up, about 40 years ago, and they were very good at low volumes. Mind you, the MX-113 tube pre had bass and treble controls, so that helped. They also had a dynamic low-frequency driver ...

High eff with low power its not hard to find low noise amplifiers.

Many thanks for a lot of inspiring input. I have enjoyed reading the responses including the recommendations for other ways to improve SQ than replacing speakers.

My selected take-away for now is
- go for high sensitive speaker (above 92 DB)
            - eventually go for Horns
            - eventually go for studio monitors (or mid size speakers) 
- add EQ in some way, either via DAC, Pre-AMP or room correction
- replace current AMP with a Tube-based AMP

I will start with the speakers and move down the list - and replace my DAC at some point. 

Many thanks. Michael

My experience shows me that good interconnects and especially an upgraded power cord (nothing crazy here. Say $50 for each set of phone plugs to get decent quality plugs and say $75 for the power cord. The Pangea cord from Audio Advisor is very good). People tend to go crazy here but there is a point of diminishing returns of dollar to sound quality. My point is being-to recover as much detail as possible which fleshes out the music at lower levels and does improve efficiency as well. Speaker placement through trial and error, assists greatly to produce a solid sound stage where getting the tweeters at ear level is important.  Smaller speakers, say between Dynaudio to Def Tech 800s are great with a quality subwoofer.