The comments made by a couple of people talking about how speaker designers moved to lower resistance/less efficient speakers in the quest for more SPL and more bass is my recollection, too. That also helped to start the monster amp movement in the late 70s which peaked in the late 80s; at that time all we seemed to care about was the amount of capacitance a solid state amp had. Tube amps were weird back then to most of us. Check out the older ADS speaker designs from the late 70s and early 80s and that would be the poster speaker for the movement to less efficient designs in the quest for moderate sized speakers with lots of bass. This is also the reason so many mini moniters require LOTS of power to drive them and make bass.
This is a quote from Roger Russell's website on the change McIntosh made to its speakers in 1993:
Changes were made in 1993
Impedance of Home Systems Was Changed To 4 Ohms
All systems manufactured prior to 1993 were 8 ohms and had an industry standard sensitivity rating of 1 watt into 8 ohms. All systems made after this date were 4 ohms and have a rating of 2.83 volts, which is actually 2 watts into 4 ohms, making them appear to have greater sensitivity. To convert to a 1-watt level, subtract 3 dB from the rated sensitivity. For example: the LS350 is rated at 89dB for 2.83 volts. This converts to the industry standard rating of 86dB/watt/meter.
In addition, the new 4-ohm impedance requires the use of heavier speaker wire to maintain the same low losses compared to the earlier 8-ohm speakers. The total DC resistance of the wire must now be less than 0.4 ohms instead of 0.8 ohms.
This is a quote from Roger Russell's website on the change McIntosh made to its speakers in 1993:
Changes were made in 1993
Impedance of Home Systems Was Changed To 4 Ohms
All systems manufactured prior to 1993 were 8 ohms and had an industry standard sensitivity rating of 1 watt into 8 ohms. All systems made after this date were 4 ohms and have a rating of 2.83 volts, which is actually 2 watts into 4 ohms, making them appear to have greater sensitivity. To convert to a 1-watt level, subtract 3 dB from the rated sensitivity. For example: the LS350 is rated at 89dB for 2.83 volts. This converts to the industry standard rating of 86dB/watt/meter.
In addition, the new 4-ohm impedance requires the use of heavier speaker wire to maintain the same low losses compared to the earlier 8-ohm speakers. The total DC resistance of the wire must now be less than 0.4 ohms instead of 0.8 ohms.