What is your take on high efficient speakers vs. low efficient speakers?

There are good and bad examples of all kinds of speakers with different attributes.  Assuming the speaker has a reasonably smooth and balanced frequency response, the efficient speaker has several particularly important advantages.  The primary advantage is that, at least for my taste, the very best amps are low in power output. I like tube amps, but, mostly those that do not use higher powered pentode output tubes that are used in large quantities to achieve high power, and even solid state amps seem to be better sounding if they are not extremely powerful (e.g., First Watt amplifiers).

The other advantage is that high efficiency speakers tend to have better dynamics because they will deliver decent volume without as much power being dissipated as heat.  That heat being dissipated in low-efficiency speakers increases the resistance of the electrical component, which means less current/power can be delivered; this is thermal compression that means the volume level does not increase in proper proportion to the signal level.  

+1 @larryi 

Your last paragraph sums it up to me…

Lack of dynamics (generally due to thermal compression) always brings me back to “HIGH SENSITIVITY LOUDSPEAKERS”

@larryi What would you say are the attributes of low efficient speakers?  Anyone?

A lot of my listening is at low to moderate volume levels.

In my experience, high efficiency speakers sound better at such levels, especially in the bass.  Low efficiency speakers seem to lose bass impact at less than high volumes.

There are many low efficiency speakers that are quite good sounding, but, that is despite being low efficiency--there is nothing favorable about that attribute.  For example, Soundlab electrostatics, Magneplanar planar magnetic/ribbon, Falcon LS3/5A small box speakers sound pretty good, but, because they are low in efficiency, they are more limited in the ability to use amps such as single ended triode tube amps or lower-powered pentode/tetrode tube amps like those employing the 6L6 tube.  Everything else being equal, I will take a high efficiency speaker any day over a low-efficiency one.

Most high efficiency speakers take advantage of very big boxes to deliver higher bass volume.  When stereo came along, it became harder to fit two big boxes in a room, which is why smaller, less efficient speakers were favored.  The use of transistors made it easier and cheaper to provide the necessary power for these less efficient boxes.  These practical considerations played a big role in low-efficiency speakers taking over.