CWRU music professor Ross W. Duffin has made his name as a scholar in several fields, among them, fifteenth-century Franco-Flemish music, English music of the Jacobean period and the study of historical tuning systems. Recently, though, he’s become fascinated with early American music, a subject he explored with the professional singers of Quire Cleveland in “The Land of Harmony: American Choral Gems from The Bay Psalm Book to Amy Beach."