Hi.
Even if You did not, I will.
IMO the most important contributing factor to the sound was the engineers and producers.
Think about it.
We still have some of those artists around.
Most if not all are still recording music.
Some have matured like a good wine, some have turned into vinegar.
Most have an access to the latest and greatest technology, as well as
all the vintage gear, often something that money can't even buy.
But even so...
Their older recordings from the 50's, 60's & 70's sound "better" for most folks.
Could be nostalgia, or the familiarity of the "old", but often the only variable that has really changed is the recording engineer (and the times, You're never too young to blame the time destroying the good in anything

).
The engineers of the yesterday were professionals, often disliking the "noise" they had to capture and process, but still doing a job only a handful of the recording engineers today in the mainstream can.
Those engineers, combined with finely honed tunes and professionalism from the years on the road IMO made the sound.
Regards
Sam