Timlub and Unsound, back in the 1950s the tube was the only game in town. Its no mystery that tube power is expensive- by the late 1950s the most powerful amps for home use made only about 60 watts per channel.
Now it should not be a matter of proof (Unsound) but common knowledge that the closer tolerances used in high efficiency speakers results in greater reactivity (reverse EMF) from said driver. Timlub, I am including you in this post as the fact of the matter is that during the 1950s the higer efficiency speakers that existed out of necessity required greater precision to manufacture.
However with the advent of the transistor inexpensive amplifier power became available. There were two things that were direct results: 1) amplifier manufacturers realized they could make more money, as the transistor amps cost less to produce, 50% less or more, yet they could charge at least 90% of what they did for tube amps of the same power. 2) speaker manufacturers realized a similar benefit; by reducing precision they lost efficiency, but the cost of the drivers fell by 90%.
Its like I said, just follow the money. The advent of transistor amps and lower efficiency speakers was not about advancing the art (although the marketing certainly made it look that way) it was about *making money*.
Unsound, if you want proof that I did not make up this idea of the Power Paradigm, look for a Fisher 55-A amplifier. Here's the Google search:
http://www.google.com/search?hl=en&q=fisher+55-A+amplifierTake a look at the fifth hit (but look at the other links too). Its a Youtube video, but in the photo provided by Google, you see a control. The control is marked 'constant voltage' at one extreme 'constant power' in the middle and 'constant current' at the other extreme. Constant power is a zero feedback state in a higher output impedance amplifier. If I made it up, how come Fisher put that term on an amplifier made the same year I was born? The simple fact is the idea existed 55 years ago, and was the state of most tube amps, and speakers had to be designed to work with them since they were the only game in town.
Such speakers like Altec, JBL, Klipsch and the like all have midrange and tweeter controls. Now most people *these days* think that those controls are there to allow the speaker to accommodate the room, but this is not true. The controls are there to accommodate the power response of the amplifier! Once you realize that fact, once I realized it, the reality of how things were done back then became more clear: it was the Power Paradigm.
This is why I recommended you look at the history of audio.