Three gems plucked from the early works of each composer: Beethoven’s Cello Sonata, its finale a vehicle for virtuosity that must have tested the limits of even the best players of the time; Debussy’s Piano Trio, sweet and sentimental; and Mendelssohn’s String Octet, lauded by critics who say “its youthful verve, brilliance and perfection make it one of the miracles of 19th-century music.”