I have a vintage Marantz and a vintage Sansui, both stereo receivers. They have main and secondary speaker terminals. This was to have your main speakers and then a remote pair in another location. Most of the units of that era did not have a design that employed multiple impedance outputs ( 4,8,16 ). So they were rated for power at 8 ohms and would state acceptable load 4-16 ohms. Sometimes we’d stack two pairs of speakers and run A and B, or wire two pair in parallel. We were just kids then , but we’d listen for distortion and clipping. New gear has a higher degree of engineering.
Vintage amplifier specifications
I've noticed when I read vintage amplifier specifications, and by vintage I mean 70's,80's maybe 90's, I do not see power output specified at 4 ohms. Did those older amplifiers not do well on 4 ohm speakers so leave that information out or was it not considered a a matter of interest to the buying public. Maybe something else?