Nearly forty years of performing has stripped the Kronos Quartet of its ability to shock. If the group continues to surprise, delight, and confound its audience, as it did during two sets last night in the Southern Theatre, the string quartet has considerable more license as it continues to find new directions. Its thematic breadth has grown richer, as well. Last night, the group played love songs, a tragic opus about 9/11, pieces about the fundamentals of music, and one filled with irony. They made them all their own, from commissioned works to several that required radical rearrangement for four performers. Steve Reich’s jarring and emotional commission WTC 9/11 included electronic embellishments. Like his work about the Holocaust, it featured a persistent throb carried by the violins of David Harrington and John Sherba. Frequently, though, the violins, violist Hank Dutt and cellist Jeffrey Zeigler played melodic snippets in unison with found recordings — of a busy phone, witnesses to the Trade Center attack, victims’ recollections and news clips. The effect was devastating as the dashed phrases suggested both shattered glass and shredded lives. Richard Wagner’s Prelude from Tristan und Isolde was unique, as it was adapted from full orchestra by composer Aleksandra Vrebalov. Scaled to four voices, the remake marvelously retained the euphoric swells while it took liberties with a passage that suggested the original’s harmonic impact on modern music. The four musicians pushed limits anew, performing on other instruments, from sitar and hand drum to an exotic collection of electronic gizmos. The second set closed with Nicole Lizee’s Death to Kosmicsche, an extended piece that not only explored a complicated structure and included an ironic edge, but employed several instruments that harkened back to the era of ’50s sci-fi soundtracks. Dutt stroked one like an old Theremin, Zeigler strummed another that looked like a plastic electric zither, Harrington bent the antenna of a tiny robot, and Sherba finished the piece by wrestling a hand-held turntable. All four worked from a score. One can only imagine the musical notation.