But for the most part I like low efficiency speakers. That's because I'm a dipole freak: open baffle, magnepans, electrostats, etc.
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You would think straining an amp as little as possible would yield the best sound
For what it's worth, electrostats are low sensitivity, but high efficiency.
If you get down to the physics of it all, you'll find that efficiency and distortion are inversely proportional. So really, a lower efficiency speaker should sound worse...
I also wanted to mention that a lower impedance speaker is harder for the amp to drive. It's also harder for the amp to maintain a flat frequency response when the speaker impedance changes dramatically at different frequencies.
However, if you're running an amplifier that has poor performance near zero watts, then a lower sensitivity speaker will force you to run the amplifier at higher power levels - which may push you into a region of higher linearity for the amplifier.
I hate to make such a broad claim, but there are often times where an amplifier designer can choose between low distortion at low power levels or low distortion at high power levels. So if you like your sound and then throw in a higher sensitivity speaker, then you gotta make sure your amp isn't causing you to think the speaker is a poorer performer. You might just be running the amp in a portion of its range that has like 5% distortion...
I have found that I generally prefer speakers with much higher sensitivities. I also find that I am extremely sensitive to resonances, which is a shame because a lot of horns (that give you way higher sensitivies) tend to have some stray resonances associated with them. However, some of the newest stuff is so "inert" (for lack of a better term), that you end up getting the best of both worlds.
You also gotta be careful about poor source material. Stuff that was intended to be played back over the radio or in sound systems is often mixed in a manner that it almost requires some power compression from the speakers. When you've got a super linear system, poor source material will just sound absolutely horrid. But at the same time, really good source material will be just amazing. This is one of those things that I've been trying to rationalize and come to grips with...mostly because there's a lot of music that I like that is recorded like c rap.