What makes a speaker sound great at low volume?

I have a pair of high end headphones and a good head amp for late night listening. The high efficiency of a speaker is usually a good indicator that it will sound good at low volumes. but you do need to audition it.As another said, getting USED TO listening at low volumes matters a lot. I agree if you listen pretty loud all the time, hear a low volume sounds bad. Where folks like myself who listen at lower volumes all the time, it sounds great!
And most electrostatic sound wonderful at low volumes. As well as high efficiency speakers.

I use the tone controls on my McIntosh C2500 preamp at low volumes.  Does wonders.

A tube preamp will help.

I am thinking of getting the Luxman 509x for my office and one of the big reasons for it is the Loudness switch. It would help big time when I am working in my office at 3AM.

While some if not most of the above comments certainly have merit, I think one over looked crucial factor is the speaker placement itself.
Many audiophiles change things before really getting the most out of what they have in the first place and speakers are certainly one of if not the most important piece of equipment.

My speaker Dynaudio Special 25,s certainly aren't the most sensitive' I found that once I learned to REALLY set up speakers the low level listening details became more aparent with low level listening much more rewarding. Low level listening for me is early mornings when I'm preparing for class, but when I get home my listening volume goes up to high 70db,s to mid 80,s and the louder you play the more crucial speaker placement becomes.