Why are high efficiency speakers preferred for low volume listening?

I’m not the best one to address this but what is your budget? I just start here because the qualities you listed could lead to a $3000 system or much higher. You get what you pay for generally speaking.

In general high efficency speakers if done right can portray the source, whatever that means, in a more natural light, driven by a wide array of textures, spaciousness, power, etc.

You can get it with low efficiency but IMO it will cost more. The room might really the amplifier that determines your higher end.

The same can be said for lower efficiency. More power, more slam!

Impedance curve is simple- All drivers get similar power, or some require much more than others. That difference is the major source of the curve.

You really have get out and listen for yourself. Travel!

Good luck brother!

Perhaps a frequency bump in the 4-6 kHz range that adds “edge” to the sound and attracts attention at lower volumes.

Same can be easily accomplished with other good quality less efficient speakers like say Sonus Faber, KEF or even Ohm using DSP or equivalent parametric eq.

I’m listening to a pair of SF Concerto Domus at low volume that way using Roon DSP as I type. Might be fooled into thinking they were high efficiency horns with eyes closed.