Part of the fun of watching Xiu Xiu is imagining that singer Jamie Stewart could break down sobbing at any moment. But the band's ill-medicated, bipolar pop swings from hesitant joy to abject grief with surprisingly little melodrama. Stewart's wounded, whispering vocals are cushioned by carefully controlled atmospheres and highlighted by unlikely song structures. His lyrics find emotional extremes in both the mundane and the surreal. Live, Stewart's energy is balanced by Caralee McElroy's cool presence on keys as well as Ches Smith's nimble drumming. (Neumo's, 925 E Pike St, 709-9467. 8 pm, $10, all ages.)