Why are high efficiency speakers preferred for low volume listening?

I've never observed any correlation between high efficiency speaker and improved performance at low levels. Rather, (again, IME) the most important characteristics to enjoyable listening at low volumes are resolution of low level detail and microdynamic shading (as opposed to macrodynamics).  These can be found, to varying degrees, in both higher and lower efficiency speakers.

With that said, at the extreme, I have noted as a characteristic that very low efficiency speakers (e.g., <84 db/W/m) tend not to be very good at preserving microdynamics.

I came across this article a few years ago and it’s worth reading/noting.  Adds some “grist for the mill” for the conversation here I think.

WHY THE SET & HIGH EFFICIENCY SPEAKER APPROACH WORKS

DEC 2020
by Steve Deckert

Matching inefficient speakers to capable amps can overcome the loss of low volume dynamics that happens to us with large speakers. 

Something I can speak first hand to. 

My wife is a nurse and works nights, sleeps during the day.  Our bedroom is directly above the listening room.  Fully half the time I listen to my system when she is sleeping.  My Maggies (84 db sensitivity) at low volume give me so much detail and even dynamics that it full out surprises me almost every time!  I've had guests comment on exactly the same surprise that such low level listening can produce such detail.  I also have two REL subs hooked up and the bass at low volumes is ever present.  I fully believe it is a mix of electronics and speakers and I cannot agree with the statement that high efficiency correlates to quality listening at low volumes - Not on my system anyway.

OP,

thanks for catching my memory lapse and I really am referring to port/no port, omni-directional, controlled directivity.

here a KEF, there a Revel, any where you go ...

you wrote

"so I was thinking that if one was low level listening with an adequate amp, the sensitivity wouldn’t be that critical."

I absolutely agree, in theory, assuming no mis-match of equipment/space/placement/frequency distribution ’received’ at listening position. which is: an impossible assumption/comparison.

my friend’s little KEF Reference 1’s needed some serious juice (spl 81, sealed, no ports), made a great 1st impression. His mono block beasts burnt out after a while. Bass only down to 90 hz admitted to in those days. He built a table with an enormous down firing sub to that system after a while, I’m afraid to remember 30".

KEF Reference 1, sealed, no port, spl 81db

Add a port (other KEF, Revel, .....), sensitivity goes up a bit, all the way up to 86db hah, and bass performance goes lower a bit, however, now you have a much higher amount of indirect secondary ..... sound waves.

you also quoted:

"they are not finicky as far as placement." (about the ported Revels).

I think, when a port is involved, any/all locations already involve so much reflected sound ....., whereas, no port, front emanation, the placement(s) make a more discernable/measurable difference, so .... ported is less finicky isn’t necessarily a good thing.

..........................................

this becomes academic, a choice has to be made for a small room, many small ported speakers sound very enjoyable with or without a sub.