Stephen Layton’s Polyphony are an always impressive choir of startling purity and clarity. This collection of 20th-century US material encompasses familiar pieces from Randall Thompson alongside Barber’s declamatory and shifting Reincarnations and several motets, concluding with his great Let down the bars, O death. Bernstein’s acerbic Missa brevis benefits from fine solo work from countertenors David Allsopp and Christopher Lowrey, and we hear a curiosity from Bernstein’s teacher, Aaron Copland: four motets written when he was a student of Nadia Boulanger in Paris, each infused with the influences of the French choral tradition and a long way from the style we associate with the mature Copland.