I've heard that R' Haim Soloveitchik said that although there is no law stating that Amoraim may not argue with Tannaim, nevertheless it isn't customary for an Amora to contradict a Tanna. Does anyone know the source to this?
See Kovetz Shiurim (Bava Basra 633).
(The book Torah Chazal and Science quotes the R. Elchonon citing R. Chaim. I imagine then that it isnt in any of R. Chaim's sfarim.)