Why are high efficiency speakers preferred for low volume listening?

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.

@immatthewj No, I was more suggesting the idea that if the speaker were 91 dB instead of 86, that the amplifier would be doing far less work. You could get the same sound pressure you do now with 1/4th the power. So your 50 Watts would be much closer to 'unlimited power' and distortion would be lower; therefore more transparent.

@immatthewj No, I was more suggesting the idea that if the speaker were 91 dB instead of 86, that the amplifier would be doing far less work. You could get the same sound pressure you do now with 1/4th the power. So your 50 Watts would be much closer to 'unlimited power' and distortion would be lower; therefore more transparent.

Ah, now I've got you, @atmasphere  .  That makes sense.  And I suppose that is what I was thinking about when I posted this question.