The term "post-rock" has become a nondescriptor, but if it applies to anyone it applies to Tortoise. The Chicago quintet comprises five virtuoso players, egalitarian in their approach, angling toward a shifting, dramatic sort of instrumental equilibrium. Yeah, it's brainy stuff, but a band this comfortable with breaking bounds can make trigonometry sound soulful. (Neumo's, 925 E Pike St, 709-9467. 8 pm, $15, 21+.)