I've always been of the opinion that the music changed come 1968/69, from that Motownesque/Stax-Atantic soul to a more funky protest type. The atmosphere of "we can do anything" got replaced during the Vietnam war by a more somber tone, something that got reflected in the music. Soul as we know it pretty much disappeared as Disco stuff started to arrive, despite the efforts of Dakar, Lizard and Invictus/Hot Wax. The record labels didn't/couldn't adapt fast enough.