It's reasonable to think that had J.J. Johnson not abandoned the bandstands of America and Europe for the soundstages of Hollywood (yes, he did do the scoring for The Mod Squad), he would still be a familiar name in even non-jazz households. Like Oscar Peterson, Stan Getz, Miles Davis, and a few others among his contemporaries, Johnson not only helped define the 40s avant-garde (bebop) for listeners of the 50s; he came to symbolize the instrument he plays. And even though his prowess on the bulky slide trombone has slipped a little--at 64, he's not quite so quick, so effortlessly forceful--he still retains most of the startling dexterity that led initial listeners to suspect he was in fact playing the more manageable valve trombone. Despite all that, and the pleasure at hearing, once again, a genuine legend in person, it's his current band that deserves the most attention. The rhythm section includes the irreproachable Rufus Reid and Victor Lewis (on bass and drums) behind Stanley Cowell, one of those portmanteau pianists who seems to sum up the instrument's history in his style. Even better, Johnson is joined up front by Ralph Moore, the British-born saxophonist whose second album (Round Trip, on Reservoir) left no doubt about his imposing, well-considered solo capabilities. Monday through next Saturday, June 25, Joe Segal's Jazz Showcase, Blackstone Hotel, 636 S. Michigan; 427-4300.