Since getting together a couple years ago, the Rembrandt Chamber Players have emerged as one of the top chamber music groups in town. Their CD (on the local Cedille label), titled 20th Century Baroque and featuring Dominick Argento's Six Elizabethan Songs, was a remarkable recording debut. The only, albeit minor, limitation of the group has been its rather unusual mix of instruments: Sharon Polifrone on violin, Barbara Haffner on cello, Sandra Morgan on flute, Robert Morgan on oboe, and David Schrader handling all the keyboard chores. But the addition of talented violist Keith Conant this season allows them to expand their repertoire to include more traditional quartets and quintets. Indeed, two pieces in this season opener call for the viola: Britten's youthfully rhapsodic (and neglected) Phantasy Quartet (1932) and Jon Polifrone's Divertimento. Polifrone, who happens to be Sharon's husband, teaches at Virginia Tech and earned his PhD from Florida State, where he studied with Carlisle Floyd, composer of the quintessential American opera Susannah. Much of Polifrone's music is tonal, with dissonant touches for dramatic effects. His four-movement Divertimento, a commission by the Rembrandt scored for the entire ensemble, is said to be upbeat, lyrical, and Bernstein-esque. Rounding out the program is Tchaikovsky's lushly romantic and melancholic Piano Trio. Sunday, 3 PM, concert hall, DePaul University, 800 W. Belden. Monday, 7-30 PM, Lutkin Hall, Northwestern University, 700 University, Evanston. 708-328-2492.