Singer Pur is a popular a cappella vocal ensemble best known for its performances of early and sacred music. But the group has also delved into Romantic-era and 20th century fare, including popular music. The 2005 album HerzTöne (Love Songs) was in fact a quite daring crossover blend: it contained songs by Mendelssohn, Schumann, Brahms, Gershwin, Erroll Garner, Burt Bacharach, Chick Corea, and Billy Joel, along with a few items by lesser-knowns and traditional songs like Danny Boy. The result was a delightful admixture of truly eclectic repertory. What distinguishes this group...