There's certainly nothing new about a band using bluegrass as a base for exploring musical territory outside of that genre's rigid boundaries: from New Grass Revival in the '70s to more recent experiments by Sam Bush and String Cheese Incident, the steady rise of Americana has owed much to these acoustic renegades. But the Massachusetts-based quartet Crooked Still, on their sophomore release, establishes themselves as one of the more notable outfits to take the post-bluegrass formula and run with it.