Complex sentence: That year, he helped organize the Monterey Pop Festival and, having championed American acts such as the Byrds and the Beach Boys since 1965, he subsequently guided the magazine's direction toward US countercultural issues and psychedelia.
Split into simpler sentences: That year, he helped organize the Monterey Pop Festival. Having championed American acts such as the Byrds and the Beach Boys since 1965, he subsequently guided the magazine's direction toward countercultural issues and psychedelia.