Gregg Allman, whose hard-jamming, bluesy sextet the Allman Brothers Band was the pioneering unit in the Southern rock explosion of the ‘70s, died Saturday due to currently unknown causes. He was 69. As recently as April 24, reports surfaced claiming Allman was in hospice. His manager previously denied those reports to Variety, which Allman then substantiated in a Facebook post. With his older sibling, guitarist Duane Allman, the singer-keyboardist-guitarist-songwriter led one of the most popular concert attractions of the rock ballroom era; the group’s 1971 set “At Fillmore East,” recorded at Bill Graham’s New York hall, was a commercial...