What makes a speaker perform well at low volume?

My experience indicates that more efficient efficient speakers usually "perform well" i.e, deliver music and detail at lower volumes.

Be especially careful when demoing horn-loaded speakers. They are usually dynamic and may sound exciting at first but many horn owners complain of listener fatigue over time due to their "in your face" response.

You need a speaker that doesn't shout and produces what is on the recording, nothing more, nothing less. And be careful with true studio monitors, they are usually uncolored and very revealing but don't have the mid range magic our ears enjoy over the long term.