Aroc asks an interesting question. I'm hardly an expert but as I've come to understand it, the first speakers were actually extremely efficient. The early years of vacuum tube amplification required efficient single driver baffle mounted speakers. Musical, but pretty crummy frequency response. Later came the "great" multidriver high efficiency horn speakers (Klipsch's, Altec's, JBL's, etc).
The less efficient sealed box designs utilizing multiple drivers and power robbing filter components happened to come at a time when we saw higher output tube and transistor amplifier designs. Efficiency was no longer a big deal. The culmination of this was probably the small closed box mini-monitors that utilized complex power hungry crossovers and a sealed box to create "bass" from much smaller than expected drivers. (I played with these designs for awhile in the early 80's. The crossovers were complex and the resistor networks in them often became quite warm to the touch at moderate listening levels. Lots of the amplifier power was used to warm the room! I recently acquired a blueprint copy of the KEF 101 crossover from the factory and that extremely complex 2-way crossover included 25 components including a relay and an LED!)
Vented box designs promised better efficiency with even lower bass. (Unfortunately it's not always well executed.) That has sort of made them the predominant design for the last couple of decades. Now, with audiophiles falling into various camps that favor everything from SET's with single digit wattage to solid state designs with nearly 4-digit outputs, speaker designers are free to create what sounds best to them with the knowledge that proper amplification can be found for nearly design.
Sean is right, unless you're running a SET amp, choose any reasonably efficient speaker. Other than that there don't seem to be an absolute pros and cons to high efficiency. If your amp is unhappy with low impedances, checking the impedance curve of a speaker is important before buying. Again, I'm certainly no expert but those are the only numbers I pay much attention to. I buy what I enjoy listening to.
The less efficient sealed box designs utilizing multiple drivers and power robbing filter components happened to come at a time when we saw higher output tube and transistor amplifier designs. Efficiency was no longer a big deal. The culmination of this was probably the small closed box mini-monitors that utilized complex power hungry crossovers and a sealed box to create "bass" from much smaller than expected drivers. (I played with these designs for awhile in the early 80's. The crossovers were complex and the resistor networks in them often became quite warm to the touch at moderate listening levels. Lots of the amplifier power was used to warm the room! I recently acquired a blueprint copy of the KEF 101 crossover from the factory and that extremely complex 2-way crossover included 25 components including a relay and an LED!)
Vented box designs promised better efficiency with even lower bass. (Unfortunately it's not always well executed.) That has sort of made them the predominant design for the last couple of decades. Now, with audiophiles falling into various camps that favor everything from SET's with single digit wattage to solid state designs with nearly 4-digit outputs, speaker designers are free to create what sounds best to them with the knowledge that proper amplification can be found for nearly design.
Sean is right, unless you're running a SET amp, choose any reasonably efficient speaker. Other than that there don't seem to be an absolute pros and cons to high efficiency. If your amp is unhappy with low impedances, checking the impedance curve of a speaker is important before buying. Again, I'm certainly no expert but those are the only numbers I pay much attention to. I buy what I enjoy listening to.