The audience present for the 8:30 p.m. Bad Plus set last evening leaned more toward curious observers than diehard fans. It was difficult to suspect the sort of crowd that would take interest in the piano, bass, and drum trio, and even harder to guess what aspect of the group appealed to whom. Some seemed to favor the band's groove-heavy material, nodding their heads when Reid Anderson's limber upright-bass figures coalesced with Dave King's propulsive beats in a way that has drawn comparisons t