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generally speakers in the early days of audio had very high efficiency because the amps of the day were low powered also the speakers were quite large. To get more bass out of smaller speakers the efficiency dropped as well. its easier to make high efficient larger speakers then smaller ones but most people don't live in theaters so speaker designers designed speakers that were smaller and took advantage of the new (70's) SS amps that had more power. But as Tube's have made a big come back and lower powered ones seem to be also coming back full force, speakers makers are once again designing higher efficiency speakers and they tend to be larger ( if you want full'ish range) but that's not always the case.