Bach might be back on Washington's airwaves, even if the region's only classical music station, WGMS, drops the format. Public broadcaster WETA (90.9 FM) is considering dumping its news-and-talk programming and returning to being a classical broadcaster if the music dies on WGMS, WETA's management said yesterday. In a special meeting Thursday, WETA's board voted to give station executives the green light to consider switching back to classical if WGMS drops the format. Dan DeVany, WETA's vice president and general manager, said the station "could move very quickly" back to classical if circumstances warrant.