Portland Baroque Orchestra sets Johann Sebastian Bach beside two of his sons to hear how far the next generation went. At the center is the elder Bach’s Triple Concerto in A minor for flute, violin and harpsichord, with Joseph Monticello, Carla Moore and Julian Perkins. Alongside it come Johann Christoph Friedrich’s stormy Symphony in D minor, and a flute concerto and the Symphony in C major by Carl Philipp Emanuel. Whether these are kindred voices or rivals, each listener can decide.