In the wake of wildly successful adventurers like Alison Krauss has come a growing variety of country subgroups, resulting in an acoustic-driven tributary that flows somewhere between the larger waterways of country, bluegrass and folk. Crooked Still diverts into a stream almost of its own, offering a blend of American roots music and skilled musicianship best described as “mountain music goes to grad school.” The initial old-meets-new surprise of the band’s debut disc has dimmed somewhat on album number four, Some Strange Country, which nonetheless boasts the approving nods of guests Ricky Skaggs and Tim O’Brien, virtual great-uncles of progressive acoustic music. Crooked Still produces a rare wine, and listeners with well-developed palates will find the virtuosic banjo, fiddle and cello playing, thrushlike vocals and quiet introspection an alluring brew. Those whose musical tastes run toward beer on tap, however, are advised to sniff the cork before filling their glass.