The classical labels differed vastly in recording quality. Many people know about the superb early RCAs and Mercurys. But there is much that was not as well recorded, for example, almost all classical music on Columbia, RCA post-1964, multi-miked DG, etc.
What I always found curious was that the Columbia jazz was well recorded but not the classical. Obviously a different set of engineers. Tragedy is that there are many GREAT performances by Bernstein on Columbia in miserable sound.
Don't forget about Nonesuch classical either. Modern American music but extremely well recorded Aubort and Nikrenz by and mastered by Bob Ludwig.
The of course, there was Decca and EMI on the other side of the Atlantic whose classical recordings were every bit as good as RCA and Mercury. Then there were the other Decca labels such as Argo and L'Oiseau-Lyre. Some esp. nice stuff by the Melos ensemble on L'Oiseau-Lyre. Sad thing is that the US Angel and London releases didn't come close to the UK releases.