What makes a speaker sound great at low volume?

Lots of things. My speakers aren’t the most effecient and require a big amp. My system sounds very good at low volumes which is how I do most most of my listening. I’ve done some tweaking to lower the noise floor which is what I think has worked for me. Start with clean power, dedicated circuits, quality receptacles, quality cables, and cable placement. Nice rigid equipment stand, isolation devices between your components and shelves. I think it’s getting rid of the noise to start with. Speaker placement, your listening position and room treatment also play a big part. Guess I’m saying it’s not just looking at a pair of ultra efficient speakers because it can be done with not so effecient speakers as well. It takes time, experimenting and not being afraid of tweaks, such as fuses and etc.