When San Francisco’s Hardly Strictly Bluegrass Festival began, nobody had heard of the Avett Brothers. Now they might as well be its poster boys. Scott Avett may play a banjo, and Seth strums an acoustic guitar, and bandmate Bob Crawford handles upright bass, but they play with a percussive energy that owes more to their punk sensibilities than to their North Carolina birthplace. To them, a holler is a throat-shredding vocal, a square dance is a manic stomp with high kicks, done from behind a microphone in, say, Nashville’s Ryman Auditorium, where they won the Americana Music Association’s Honors & Awards’ inaugural Duo/Group of the Year Award in 2007, along with New/Emerging Group of the Year. Both were in recognition of their breakout album, Emotionalism — the one that captivated Rick Rubin, who signed them to his American Recordings label. The result is I and Love and You, released in July. With their brilliant lyrics and captivating deliveries, and Rubin’s unerring sense of direction, it won’t be long before genre definitions fall away completely.