At age 20 the youthful-looking Kronos Quartet still doesn't quite have the technical prowess of a great string quartet, but its devotion to multiculturalism and political causes certainly marks it as a galvanizing musical spokesman for the 90s and beyond. One of a handful of crossover successes, the Kronos, who these days visits the city about once a year, has won over a boomer following that is the envy of establishment institutions such as the Chicago Symphony. Much of its appeal, of course, can be attributed to the foursome's calculatingly hip demeanor, choice of venue--and foundness for theatrics. For its latest local appearance the Kronos will mount a multimedia presentation of Black Angels by George Crumb, the iconoclast whose key works of two and three decades ago are now seen as compelling statements of the angst and experimentalism of the Vietnam-era counterculture. Written in 1970 during the darkest days of the war, Black Angels depicts with vividness harrowing battleground and protest sounds. Hallucinatorily intense yet eerily meditative--aided by the use of classical allusions and an arsenal of shouts and whistles--it is the musical equivalent of Apocalypse Now. The top half of the Kronos's program is a salute to contemporary music's cultural diversity and protest politics, including "Escalay--Water Wheel," a lament against western encroachment by the Nubian composer Hamza El Din; Osvaldo Golijov's 1992 "Yiddishbbuk," a miniature amalgam of Jewish and other folk traditions; Scott Johnson's "How It Happens," a tribute to I.F. Stone; and Michael Daugherty's self-explanatory "Sing Sing: J. Edgar Hoover." Saturday, 8 PM, Vic Theatre, 3145 N. Sheffield; 559-1212.