The Grateful Dead spawned not one but two jam band movements. Its electric, psychedelic rock and roll road was followed by Phish, Widespread Panic and others. But the band regularly unplugged in tribute to its jug band and bluegrass origins, creating a template for freewheeling acoustic bands like New Grass Revival and Greensky Bluegrass. Leftover Salmon, among the most eclectic and beloved veterans of jamgrass, turned 30 years old on New Year’s Day.