If the only alternative bluegrass band you’ve heard is Nickel Creek, you might think that genre-busting extremes define the style. Not so. Crooked Still, another group of radical young innovators, has been reinventing bluegrass in far more subtle ways. Tonight the band celebrates the release of a new album, Some Strange Country. Opening is Gabriel Kahane, a singer-songwriter unusually well-versed in historic antecedents yet fully in step with the moment; he can break your heart with a poignant observation.