What makes a speaker sound great at low volume?

Buy a pair of Quad 57's! Either in good working condition or refurbished. They will provide all the air and detail at low volume that you could desire! You will be very happy! I own two pairs! And a good amp match for them would be a Quad 303 (I have one, along with a 405 and a rare pair of the 50 Mono's).

I love the sound of my Ref 3A de Capos at low volume detailed

using with a 4wpc decware amp

You should go listen to any LS3/5A type of speaker. Spendor, Harbeth, Stirling , etc., etc. make this design!

My former Krell 600 - Dynaudio Consequence combo was hopeless on low volume. It would have been better to listen to a small bookshelf system. It was only on higher volume that the wolf came out of the lair, so to speak. Music started sounding great, and even glorious, but only at very high volume. My present Atmasphere MA-1 - Audiokinesis Dream Maker combination offers better sound at lower volume. Even though I could have asked for even more, in this respect. Here also - even if these amps run fairly "flat" in class A - I find that to get the sound right, I need to turn up the volume. I think that getting more sensitive speakers is the way to go, but it is not the only answer.

Properly designed loudness controls, Schitt Loki, or another eq ( in the tape monitor loop ), would answer your concern. Headphones are another route to take. I am not a scientist, but been in this hobby for so long, these are my experiences. Speakers load a room differently at various volume levels, no matter what equipment you have, and our ears are quite unique instruments. Happy Fathers Day to all the pops out there ! Enjoy ! MrD.