For more than four decades, the principals of the Boston Symphony Orchestra, one of the leading American orchestras, have performed together as a chamber ensemble in different configurations. For their first appearance at the Pierre Boulez Saal, they are joined by American pianist Garrick Ohlsson in a program that includes two of the most beloved chamber music works of the early 19th and the 20th centuries: Beethoven’s Septet for clarinet, horn, bassoon, and strings and Poulenc’s Sextet for winds and piano. Opening the concert is Plain Song, Fantastic Dances by American composer Michael Gandolfi, a piece premiered by the Boston Symphony Chamber Players in 2005.