Once upon a time, a string band meant hillbilly music as advanced by Roy Acuff, Charlie Poole and John Hartford. But the Grateful Dead and outdoor-festival culture changed all that. The Colorado-based acoustic rockers of Yonder Mountain String Band have long known who now calls the tune: snowboard bums, collegiate bro-grassers and summer-fest freaks. That demographic has transformed it into the quasi-string band equivalent of the Dave Matthews Band, a massively successful touring operation that cranks out live recordings faster than even its own instrumental breaks. But like DMB, YMSB is fueled by musicianship and a groove channeled into warp-speed bluegrass — and the band has enough of a connection to country roots that even skeptics may want to ring in the new year with a hyper-driven hippie twirl.