Why are high efficiency speakers preferred for low volume listening?

low volume is a separate issue

and there is no getting around the established facts that at the least low bass needs to be boosted at low volumes, ideally 'progressively boosted' as volume lowers.

Where's Wally? Where's Ray?

Play some Jazz, Ray Brown with others or his trio. Lower the volume, ... lower some more, where's Ray? For me, it is maintaining the Bass Player in Jazz that maintains involvement when listening at low volume. Otherwise it quickly becomes un-involving background music.

No matter how much we want to avoid the truth regarding 'loudness compensation'' (because so much modern equipment lacks solutions for it), success depends on both understanding and being equipped to solve the requirement.

you youngsters: get thee vintage features:

"Amplifiers often feature a "loudness" button, known technically as loudness compensation, that boosts low and high-frequency components of the sound. These are intended to offset the apparent loudness fall-off at those frequencies, especially at lower volume levels. Boosting these frequencies produces a flatter equal-loudness contour that appears to be louder even at low volume, preventing the perceived sound from being dominated by the mid-frequencies where the ear is most sensitive."

"Loudness compensation boosts low and high frequencies when listening at low levels so that the ear perceives an overall flatter (substitute accurate for flatter here) sound pressure level. This helps quiet sounds be easier to hear, and prevents loud sounds from overwhelming the listener."

I frequently see high efficiency speakers suggested for those who listen at "low levels." And is this another area that actually is "how easy the speaker is to drive (as related to its nominal impedance)" that is more important than the actual sensitivity number?

@immatthewj Efficiency (sensitivity) is important but so is the impedance and phase angles the amplifier must deal with.

All amplifiers make more distortion into lower impedances. This distortion is audible- don't fool yourself.

So if you want to get the most out of your amplifier dollar investment, its best served by a speaker of 8 Ohms or more, if high quality sound is your goal. Its a pretty weak argument for 4 Ohms just because some amps can play only 3dB louder (which isn't remarkable to the ear).

Its harder to find high powered amps that actually sound like music as opposed to electronics. For that reason higher efficiency is good whether played at a lower volume or not since it can take advantage of lower powered and (hopefully) more musical amps. Its that first Watt of most amps that is usually the most important so if your amp has a good first Watt, you can see that a speaker of higher efficiency will be more able to take advantage of that.

FWIW Dept.: My speakers (Classic Audio Loudspeakers) are 98dB, 16 Ohms and flat to 20Hz. They are also some of the most revealing speakers I've heard.

Thank you for the input, @atmasphere ; the speakers that I am auditioning (and that I have about 3 more weeks on that audition) have an sensitivity rating of 86, a nominal impedance of 8 ohms, and according to Erin’s Corner a minimal impedance of 4.69 ohms. One of the reasons I decided to give them a whirl was that the reviews said (despite the sensitivity rating)  "easy to drive" and although I do not completely trust reviews, I don’t see why they would lie about that.

Anyway, I don’t feel that my amp (Cary V12) has been straining in 50 wpc triode mode, and I did play around with 100wpc ultralinear, but I seem to prefer the sound in triode more.

The reason I was inquiring is that second guessing myself is one of my many flaws.

@immatthewj  IME its best to have the amplifier loafing for a living. They will produce the best sound they can that way and its always audible- smoother and more detailed (as well as measurably lower distortion, which is why 'smoother and more detailed'). 

@atmasphere  , well, it does not seem to sound like (to me, anyway) as if it is straining, but by loafing, are you thinking that it would prefer being in 100 wpc ultralinear versus the 50 wpc triode mode?

I feel that my preamp (Cary SLP 05) has a lot of gain (and probably some of that is because I am using the balanced ins and outs) so I rarely move the volume knob  much past 9 oclock to achieve my desired listening level, but I realize that this does not necessarily mean that the amp is not working harder than maybe would be optimal.