Last summer, the octogenarian Sir John Eliot Gardiner struck a singer backstage and parted ways with his longtime associates, the English Baroque Soloists and the Monteverdi Choir. Another Baroque star, Christophe Rousset, has replaced him on the podium for this program. Gardiner, however, is beating Rousset to the punch with the same program, on the same stage, a week before. The centerpiece is a midnight mass for Christmas by Charpentier, bookended by a pair of Bach cantatas. For sharp-eared music lovers in Hamburg, the A/B comparison should prove highly illuminating. —Matthew Gurewitsch