Not many jazz musicians actually deserve the descriptor "living legend"--that is, besides J.J. Johnson. At the dawn of the bebop era, Johnson confronted a formidable task. Such giants-to-be as Bud Powell (piano) and Dexter Gordon (tenor sax) had begun to adapt the innovations of Parker and Gillespie to their instruments; but Johnson's instrument was the slide trombone, best known for sweet ballads and swing-band special effects and seemingly resistant to the high-speed exactitude of bebop. Johnson single-handedly lifted the instrument to bebop's high-water level of virtuosity with a technique that made early listeners think it was some sort of trick and an imperturbable cool that made it sound easy. When Johnson returned to performing in public a few years back after a two-decade layoff, the technique and the cool remained intact, giving his work a dry and distanced quality. So the arrival of last year's Let's Hang Out (Verve) came as an all-the-more-welcome surprise; spilling over with musical enthusiasm, it displays not only his stunning perfectionism but also a renewed emotional connection to the music, plus a few new tricks. In person, Johnson generously shines the solo spotlight on his accompanists; with a group as good as this one, who'll complain? It includes a lively and progressive pianist in Renee Rosnes; a paladin of the bass fiddle in Rufus Reid; yet another crackerjack young drummer in Billy Drummond; and the excellent tenor saxist Doti Braden, who gets overlooked in the shuffle of his mid-20s contemporaries but who plays with a controlled fire that many of them would do well to emulate. Friday through Sunday, Joe Segal's Jazz Showcase, Blackstone Hotel, 636 S. Michigan; 427-4846.