Over the past several years Mississippi Heat has evolved from an earnest but enjoyable revival outfit into a full-fledged contemporary Chicago blues aggregation. Their Delta-to-Chicago sound combines elements of tradition with up-to-date aggression, and they're savvy enough to feature soloists whose love of that tradition never hinders their enthusiasm for house-rocking boogie. Guitarist James Wheeler honed both his melodic sense and his instincts for soul baring during his stint with Otis Rush; both he and fellow fretman Billy Flynn combine taste with intensity in a way that's all too rare these days. Pierre Lacocque's harmonica warbles and swoops in the great postwar tradition. The best thing that's happened to this band recently, though, has been the addition of vocalist Dietra Farr: with a soul singer's depth of timbre and a blueswoman's passion for honest expression, Farr brings an exciting new dimension to what was already one of the more versatile and proficient blues units around. Her sassy charm perfectly complements the worldliness of the band's sound, and like both Wheeler and Flynn she combines irrepressible enthusiasm with a musical maturity far beyond her years. Friday, 9 PM, Koko Taylor's Chicago Blues, 7 W. Division; 337-2583.