Here we go again. One thing we modern folk oft do is to use the WRONG terminology. 'Efficiency' is different than 'sensitivity'. Speaker-system efficiency normally is measured 1 meter from the speaker/driver when driven with exactly 1 watt of power measured into the speaker's rated impedance. IOW, a '4-ohm' speaker system's output is measured while being driven by TWO volts into 4 ohms (which equals ONE WATT). Decades ago, with the increasing popularity of solidstate amps*, the audio industry started publishing sensitivity ratings, and not all of them do it correctly.
Sensitivity is measured with a signal created by driving an 8-Ohm load resistor with 2.83 volts. That does happen to be 1 watt into 8 ohms, but it's not 1 watt into 6 or 4 or 3 or 2 ohms. The correct way to state this is 'sensitivity at 1M using 2.83v. into 8 Ohms' or something like that. And BTW, it's not PER meter, it's at 1 meter.
*--Solidstate amplifiers are constant-voltage devices and not constant-current devices are as most tubed amps. That is, because a SS amp's output impedance is so low, it will maintain its output voltage into decreasing loads until it reaches its maximum current output. A tubed amp generally will NOT increase its current output much as the load impedance decreases.
.
