For soul music’s greatest artists, the ’70s meant reaching new ambitions and new levels of sophistication. Freed from the strictures they may have felt in Motown’s ’60s heyday, Marvin Gaye and Stevie Wonder made masterful music that confronted the troubles they saw in the world while pushing soul into uncharted territory. Meanwhile, innovators like Philly soul kings Kenny Gamble and Leon Huff expanded the sound’s scope, resulting in sumptuous string-laden hits that still boasted irresistible grooves. Of course, Al Green, Roberta Flack and Donny Hathaway showed how some songs required the very softest of touches.