I can't say for certain, but my speculation is the advent of Dolby noise reduction. Recordings from the late 50s and early 60s are stunningly lifelike with mScaler
I think you definitely have made a very valid point there .
But I would also say that with labels like EMI or Philips who as far as I know did not use Dobly, can sound stunningly clear open and realistic in all the important aspects that really count for me with acoustic music: Timbre, tonality and coherent soundstage with 3D like good depth well into the last gasping days of analogue in the late 70s.
This week I am listening to Karajan's Tristan from 1972 on LPs and I can only say that few digital recordings in my collection are as lifelike as those mainly excellently balanced, half the number of mics compared to "the competition" ie DGG during the same period, to quote the balance engineer Wolgang Gülich himself.
He was responsible for some of the most realisitic sounding recordings EVER made of the BPO/Karajan in the early to mid 70s.
Clearly better than the Dolby DECCAs from the same period.
But to enjoy some of those gems one needs to listen via a system capable of playing at very loud levels without obvious effort.
They are not overly compressed like a lot of the things one can easily enoy via computer desktop speakers many seem to prefer here .
I am talking about large scale Operas by Wagner and Verdi and symphonic works like the Sibelius symphonies.
The kind of music and recordings that really "sort the wheat from the chaff" imho.
As far as Rob's take on the DECCAs I suspect that some of the hardness or edge he has mentioned is more related to inferior early ADCs digitization of the analogue masters than the actual recordings.
On LP I rarely hear any edge on DECCAs both before and after Dolby treatment. If anything Dolby introduced a softness and more blurriness to the sound.
But the few DECCA rbcds I had sure had an edge to them more often than not.
And the EMI rbcds I tried also sounded clearly inferior to my LPs.