The St. Lawrence Quartet, a Canadian group founded in 1989, has earned a reputation for spontaneity and informal persuasiveness. Their latest CD, of three Shostakovich quartets, stresses the music's humanity; it's less menacing than some recordings, and the playing--anchored by cellist Christopher Costanza, a former member of the Chicago String Quartet and the Chicago Chamber Musicians--is vibrant and never top-heavy. This concert was supposed to consist of Franck and Beethoven quartets, but the first violinist is bowing out to attend the birth of his child. The remaining members will become a trio, playing Dohnanyi's Serenade in C Major, op. 10, an early-20th-century piece that looks backward rather than forward. They'll be joined by pianist Anton Kuerti, who's given to eccentric but compelling interpretations, in a performance of Mozart's Piano Quartet in G Minor, K. 478--a real joy--and Schumann's lyrical Piano Quartet in E-flat Major, op. 47. Musicologist Stephanie Ettelson will give a free lecture at 2 PM. a 3 PM, Pick-Staiger Concert Hall, Northwestern University, 50 Arts Circle Drive, Evanston, 847-482-1714, $30, $20 for ages 40 and under, $12 for students.