best overall speakers at low volume?

I agree with Dweller and Zmanastronomy. At low listening levels, it has to do with the quality of the attenuator in your electronics (integrated or pre-amp) as to how much detail will be delivered to your amp section and speakers.
When it comes to speakers, the simpler the crossover (or even better, no crossover) such as the full range drivers in the De Capo's tend to reveal more.

Agreed with others that having a good system is important. I have the Gallo Ref 3.1 speakers with tube gear and VPI Classic table and sounds wonderful at night when everyone is asleep.

I would also agree with the above responses but weight the speakers a little higher in their contribution to this. I have recently heard the Wilson Sophia 3 in Tampa at AV South being driven by Ayre integrated and DAC and Computer front end. Don't know how much each piece was contributing to the sound but it was amongst the best I ever heard at low levels while still having amazing detail, clarity and dynamics. I've heard Wilsons in many setups, but this was best I've heard them. all this at a level that would easily allow conversation with someone in the room. Also, the system as a whole-while not exactly chump change would not break the bank either for the quality it delivered. Just a thought....

There is not one no speaker sounds great too low DUDE!!