As the pianist in Johnny Griffin's quartet, Michael Weiss has never failed to impress, exhibiting a powerful technique, a sure familiarity with music written before he was born, and an ability to interpolate musical quotations that almost matches that of his boss. But last year, Weiss stayed in town to lead a trio on his own; the results were revealing--and spectacular. The different setting and repertoire produced a more expansive player, a more graceful melodist: a more developed musical persona. (On his own, Weiss proves an imaginative orchestrator, one who manages to make the musical texture itself into an element of swing--as if texture had its own rhythmic weight.) When he approaches the pop standards and classic jazz tunes that are his canvas, he combines the wonder of spontaneity with a workmanlike acknowledgment of his materials: art and craft in delightful balance. The piano trio, by virtue of its economy and precision, has long held a special place in jazz; Weiss knows this, and you can hear it in every phrase. Tonight and Saturday, Pops for Champagne, 2934 N. Sheffield; 472-1000.