Radu Lupu is, with some justice, considered to be one of todays premier pianists. His program for Cliburn Concerts in Bass Hall on Monday night reinforced that idea.At the same time, he may have surprised a portion of the large audience that sat in rapt attention. Anyone who expected pianistic thunder and lightning instead heard a program full of subtlety and understatement. Lupu was in a reflective mood, and that lasted from beginning to end.He opened with four Schubert Impromptus, D. 935. These are lovely works that one hears all to infrequently around here. Despite their lyric beauty, there is room for drama, but Lupu passed on that, opening gently, with a whispery left hand, and keeping the atmosphere serene. Even the final impromptu, in which the tempo quickened, remained understated.The only drama was in an accidentally amplified voice from somewhere backstage. Lupu proceeded placidly.Francks Prelude, Chorale and Fugue offered the possibility of a grand finish, but here too moderation was the key. Again, there were some noises of an unidentifiable nature in the balcony area, and again Lupu ignored it.The second half of the program was given to Debussys 12 Preludes of Book 2. Given what had gone before, the audience was by now surely tuned to Lupus wavelength, and he delivered. Some of the pianissimos were so soft that you werent really sure whether you actually heard them or simply imagined them. Even the final Feux dartifice, or Fireworks, burned softly in comparison to the norm.Still, it was all gorgeous and often atmospheric. La Puerta del Vino and Brouillards were high points.