@wolf-garcia: I appreciate how you differentiate between a jam band and a "studio" band. The latter tend to play songs live as they were performed on the original studio recording, treating instrumental "solos" as song parts, not as an excuse to do extended, long flights of fancy.
When Clapton made his pilgrimage to the Big Pink house, he says he was sitting around, waiting to jam with The Band. Robbie Robertson finally told him "We don’t jam."
Rock bands and solo artists starting feeling the need to prove what good musicians they were (in terms of being able to solo well) during 1966 and 7, Cream and Hendrix of course leading the way. The Beatles remained a "song group", and the groups and bands I was attracted to were those whose members included superior song writers first and foremost.
My taste in musicians runs to those who play "In service to the song." That’s a skill much different than being able to solo on the level of a Jazz musician. Dylan started recording in Nashville in 1965 because of the way that town’s studio musicians could play a song. Another such town was Muscle Shoals, home to the likes of The Swampers (the superlative house band at Jim Hall’s studio). The more efficient route is go straight for the solo songwriters, who often have great taste in musicians.