What makes a speaker sound great at low volume?

Crossovers are the most ignored, but most important consideration, IMHO. I recently went through the same process you’re going through now. I ended up with the 2 way Joseph Audio Pulsars, which provide everything I need ... great crossover (a Modafferi based and JA modified infinite slope), fabulous sound at low and high volumes, low distortion, and great looks. Couple them with a good sub and you’re good to go. BTW, I went from full range B&Ws to the 2 way Pulsars, and can say that I much prefer the more coherent sound of the 2 way with a sub than the full range speakers. 2 way speakers can "disappear" in a way that full range can’t. Of course, your room size will have a major impact on which way to go as far as speaker size is concerned.

kalali is correct, omega speakers have 98db sensitivity, like my rs8 supercone, high output alnico 96db ? Then get a nice tube  power amp.

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!