One of rock's great voices meets one of country's great voices, tackling an inspired, roots-minded songbook with sonic guru T-Bone Burnett. Krauss sounds bluesy, even lusty; when she sings "I could never kill a man" on Gene Clark's "Through the Morning, Through the Night," you’re actually not 100 percent sure. Plant, meanwhile, dials it down so far, it’s often hard to identify the dude who sang "Black Dog." Yet the nuance in his vocals is as impressive as it is unprecedented. Tender, haunting, brilliant.