Over the past two years, Yonder Mountain String Band, Saturday, December 30, and Sunday, December 31, at Boulder's Fox Theatre, has achieved the kind of success that other new-grass acts rarely gain. In recent months, the band has played its "fully resinated" music to large, giddy crowds and headlined some of the better venues in the nation. Proof of its local drawing power can be seen in this pair of shows, which fill the Fox's revered New Year's Eve slot. The band owes much of its miracle growth to a smart combination of road-hog touring, stinging chops and a love for the extended jam. But it sets itself apart from its jammy peers with an all-acoustic, drumless approach. The result is fast-driving, banjo- and mandolin-powered music that even Kentucky purists could love. Provided, of course, they can handle stringed sessions that can reach the sixty-minute mark.