Car stereo good. One factoid from the 1960s: back in the days when Motown still was using the old "Hitsville USA" studio, one critical piece of gear they were VERY reliant on was a crappy oval speaker mounted in a beat-up wood box. But that was not just any old speaker...it was a part that GM typically used in their car radio installations back then. Remember: this was when AM "boss radio" was the thing...BIG rock and r&b signals all over, sometimes nationwide at night, and that crap speaker box was designed to make DAMN sure that those Motown hits would sound perfect when they came out of your dash. And no need for stereo, either...just that BIG, BOOMING soul sound...
Much easier than the "radio check" system used back in the old days in Memphis. As an example of that, when Sam Phillips got done cutting Elvis Presley's "Hound Dog" on a test acetate over at Sun, he ran it over to WDIA to see what sort of turmoil it would cause. Basically, if it sounded good on WDIA, then the station's phone lines would blow up. And they did. You had a lot of the labels there doing that...Stax, Hi, and so on, and quite often back in the years from the mid-1950s up thru the early 1970s, Memphis listeners often got to hear music history...before it was music history!