Some great responses above, and here's another point to consider. Many solid state desingers will not admit it, but quite a few designs sound better in their 1st 20% of power delivered, than in the next 80% left. In many ways this makes perfect electrical sense. One example: There are class A/B amplifiers that actually run in full class A up to a small point of the power amps rating, then "switch over" to full class A/B as they are required to deliver more power. If you had a speaker that was extremely efficient, you would then be having your amplifier run mostly in it's better sounding Class A state i.e., ending up with a better sounding system.
low efficiency speaker vs. high efficiency speaker?
What are the advantages and disadvantages of high efficiency speaker vs. low efficiency speaker? Does it mean that 93db efficient speaker will sound better than 86db?