Milford Graves and Jason Moran were listening hard at the Big Ears Festival on Friday evening, and in this they were far from alone. Their spontaneous musical dialogue, onstage at the elegant Bijou Theater in Knoxville, Tenn., suggested a merging of the ancient and the ultramodern, aglow with an ephemeral sort of grace. At one point, Moran's deep, mournful sonorities at the piano led Graves toward a murmuring hush at the drums, as if anything else would break the spell.