Here the speaker imagines that he can hear the footsteps of poets from the past. We think this is a really cool image for the traditions of poetry. In a way, these "grand old masters" become like ghosts in the attic. You can hear them, far away, but you can't quite see them.
Line 19: We think the idea of the past is an important part of this poem, and this might be the most vivid image we get of history. In a kind of spooky way, the poets of the past come alive here and haunt the speaker. We're not surprised that he doesn't want to hear their poems, and he seems a little creeped out to imagine them out there, walking around.