Timeless traditions and contemporary anxieties clash musically and theatrically in Leonard Bernstein's powerful but rather neglected "Theatre Piece for Singers, Players and Dancers," The dancers are omitted in this semi-staged performance, produced for the Vatican's Jubilee 2000 celebration, but the show's eclectic musical brilliance, its adept mingling and clash of musical styles, its contrasts of hope and anxiety, faith and unbelief, ancient serenity and modern dissatisfaction are all brilliantly conveyed. More than 30 years after its premiere, for the opening of the Kennedy Center for the Performing Arts in Washington, D.C., Mass still has the impact of this morning's newspaper headlines.