What makes a speaker sound great at low volume?

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.

@mrdecibel I agree . Hi end vintage gears was equipped tone control
loudness control etc, so this is problem with all hi end which you
see on the market now. Is very rare to see all new amp, with any
tone adjustment,  so this issue is depend from speakers . Sensitivity is
absolutely no important. take a look on the freq . responce---
if it is go  slightly up in midbass or bass   it will be play good on the
low volume,