I think perhaps your sample size is too small, Sonic. The 70's is known as the golden age of receivers, and rightly so. Many of the good examples from brands like Marantz, Sansui, Pioneer and Yamaha had a rich warm tone and had power of 40 to 70 watts per channel and up. At the end of the 70's there were models of up to 120 wpc.
I did some research after buying a 1968 Sansui receiver(4000) that made my jaw drop repeatedly. It is clearly superior in 2 channel than my Onkyo TX SR876. I still can't believe it. Far better imaging and soundstage with better bass. I've never heard my Paradigm Monitor 7's with so much bass, and the Sansui is only rated at 45 wpc. Way better tuner as well.
I did some research after buying a 1968 Sansui receiver(4000) that made my jaw drop repeatedly. It is clearly superior in 2 channel than my Onkyo TX SR876. I still can't believe it. Far better imaging and soundstage with better bass. I've never heard my Paradigm Monitor 7's with so much bass, and the Sansui is only rated at 45 wpc. Way better tuner as well.