What makes a speaker sound great at low volume?

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,

"What makes a speaker sound great at low volume?"

The "Loudness" switch on my McIntosh C22 preamp...

I use the tone controls on my McIntosh C2500 preamp at low volumes.  Does wonders.