Well, not ALL studio monitors were "boxes." It was kind of well known in the 1970's that SOME bands that were meticulous about every note did their final mix-downs on Magneplaner speakers.
I guess, like in the 1960's where music was mixed down to sound good on a car stereo, some bands were interested in the pure sound the public would hear.
AND, since car stereos were mostly mono AM units in the day, it is pretty easy to listen to an old 45 and figure out what was going on with the mix-down.
Cheers!