One thing I wanted to add to my post on the different attitudes to stereo mixing in different types of music in the '60s: one thing that's interesting is that the more power producers had to actually make the music, the less interested they seemed to be in stereo.
That's a big generalization, of course, but the biggest advances in stereo, and the most wholehearted embrace of stereo by producers, were in classical, where the producer cannot change the music at all. He can influence the interpretation of the music, and the way the music is recorded, but all the notes are written out already. So one of the ways for a classical producer to put his own stamp on the sound of the record was to exploit the possibilities of stereo and try and create the illusion of width and depth on the recording. (In a recording-session conversation with Benjamin Britten, his producer, John Culshaw, can be heard asking him to arrange the instruments for a good stereo layout; Britten doesn't accept that particular suggestion, but it was something that was on both of their minds.) And in an opera, the producer could literally "produce" the recording by having the voices move across the stereo stage.
In pop, and especially the album-centric pop music that developed in the '60s, the producer can and often does have a big influence not just on how the music sounds, but what the music is. A lot of, say, George Martin's job was composing or arranging, not just "producing." And as producers make music, layer in new sounds and instruments, and try to literally create a new composition in the studio, stereo could be seen as a nuisance, or at least a secondary thing. (The most famous example of a pop producer who didn't like stereo was, of course, Phil Spector, who felt that stereo separation ruined the multi-layered effects he was trying to create.)
The documentary The Golden Ring, about the recording of Wagner's Ring cycle, gives a fairly good illustration of what a classical producer (Culshaw) does. When the Funeral March has been recorded, at the end of this clip, the producer sits down with the conductor to look at the score, discuss tempo/balance, and figure out how the music will work best on the record. The documentary caused a minor scandal in the classical music world by showing that the conductor, Georg Solti, actually took advice from his producer on what tempo to use. As the documentary explains, Solti wants to take it slower; Culshaw wants it faster, and it's the fast version that goes on the record.
In fact, most studio classical recordings appear to be like that in some ways -- because the artist doesn't necessarily know how a sound that works in the concert hall will translate on a record, they often do take the producer's advice about which version will work the best. But still, the producer is only influencing the way the music sounds; he's not actually creating the music the way pop producers were doing at this time (1964). That's even more the case now that most classical recordings are made using live concerts for some of the takes (because it's too expensive to take a whole recording into the studio, and recording techniques have developed enough that engineers can make live recordings that don't sound too distant). The producer can record the concerts and rehearsals, and call a special session to fix things that went wrong during the concert, but he has fewer opportunities to influence the performance; he's basically there to record a bunch of takes and then figure out how to make a coherent recording out of them.
The documentary is in mono, which makes it inappropriate for a post about stereo recording, but I'll post a clip from it anyway, as the most famous example of a classical producer in action.