The Robert Mann Chamber Music Series continues by highlighting musicians from the Festival’s own ranks. Britten’s Phantasy Quartet begins with a march and proceeds with lively pastoral charm. Poulenc is a composer known for his aesthetic of irrepressible good humor; he called his jaunty but complex Sextet “an homage to the wind instruments I have loved from the moment I began composing.” Brahms’ sophisticated Second String Sextet is rich with nostalgia, but sweeps all sadness away in its warm, bright finale.