best overall speakers at low volume?

I agree with the Reference 3A de Cappos. I've had a pair in my system and compared them with my Triangle Comete ES and as far as low level listening, they were equal.
The Triangles are much more affordable and a little more focused. The bass on the 3A's had more fullness.

Well I have vinyl and tube gear and would urge you to try to listen to electrostatic speakers. The reQuests and my Summits are superb at low level resolution and retain the clarity and coherence at low volume. My friend's maggies were also nice at 80-90 db level. I think the dipoles help retain the "fill" at low volume.

I am historically a boom and tizz lad and upgraded and bought summits for higher volume and the better slam and headroom but they retain their strengths at low volume.

Try to audition some planars and stats and have fun.

3rd vote for Reference 3A de Cappos.

I've learned that it's not your speakers that must sound good at low levels but your electronics.

If you don't have a first class "stiff" power supply,
your sound will turn to mush.

I used a first rate integrated amp that maintained it's sonic character all the way to number-1 on the volume control. It sounded exactly the same (as higher volume)
only "less loud".

I used a TAS amp/preamp of the year that sounded like crap unless the volume control read 20 or above (then it still didn't sound good -with my speakers).
I suspect this combo used global negative feedback where the integrated did not.

So it really isn't only the speakers that make the difference.

Agreed. Electronics matters more in this case. It has been my experience as well.