One night only, step right up, ladies and gents, for perhaps the most oddly instrumented "group" in jazz. Dwike Mitchell plays piano; Willie Ruff plays French horn and bass (though not at the same time), and for 40 years they've managed to make this unbalanced tripod stand up. Give much of the credit to Mitchell: his voracious technique turns the piano-bass duets into paragons of that format; and when his partner picks up the horn, Mitchell's stylistic elegance and keyboard ingenuity remind us that the piano can act as a miniature orchestra. Ruff, who swings both his axes impressively, shuttles between lead and supporting roles with the ease of a veteran repertory actor. I've heard these guys only once, some years back, and I distinctly remember the way in which they defined their space, then filled it, and ultimately closed it off--until I could imagine another instrument only as an imposition. The Mitchell-Ruff Duo have only an indirect connection to bebop pioneer Charlie Parker--formed in the year of his death (1955), they made their most famous recordings in conjunction with Parker's partner Dizzy Gillespie--but they nonetheless will kick off Charlie Parker Month in Chicago; their one-night stand follows this weekend's sets from the stellar quartet led by vibist Milt Jackson (who did record with Parker), with drummer Roy Haynes and alto saxist Phil Woods scheduled later in the month. Monday, Joe Segal's Jazz Showcase, Blackstone Hotel, 636 S. Michigan; 427-4846.