One can debate the premise of the old joke about Mendelssohn--that he was born a genius and died a good composer--but not that his octet for strings is a work of young genius. Written when he was just 16, it was both inspired and groundbreaking, integrating the voices of all eight instruments--four violins, two violas, and two cellos. He noted that the work should be "played by all the instruments in symphonic orchestral style," and it's easy to be swept away by the exhilaratingly powerful musical current, though close listening to the complex writing for the instruments has its own rewards. Late in his life Mendelssohn spoke of the pleasure he took in writing this piece, and in all the performances I've heard, regardless of the interpretation, the musicians have always appeared to be having a great time. This performance by two young quartets should be expressive as well as spirited. The Pacifica Quartet's recording of the complete Mendelssohn quartets is vibrant and spontaneous sounding, yet thoughtfully crafted, stressing the individual voices. The Avalon String Quartet, championed by Isaac Stern and the Juilliard Quartet, made a promising debut recording of Ravel and Janacek, though it has since replaced its violist and cellist. The Pacifica will also play Janacek's Quartet no. 2, the Avalon Schumann's Quartet in A, op. 41, no. 3 (the two ensembles were originally scheduled to play quartets by Elgar and Rorem). Musicologist Patrick Casali will give a free lecture at 2 PM. Sun 5/28, 3 PM, Pick-Staiger Concert Hall, Northwestern University, 50 Arts Circle Dr., Evanston, 847-482-1714, $28, $12 for students.