According to Stevens, the setting of a poem like this is supposed to be inside the human mind. But he also throws us these images of a symbolic stage where the modern poem spends most of its time. From lines 3 to 24, you're looking at a symbolic stage where the modern poem struts its stuff.
All we really know about the setting itself, or the "stage" of poetry, is the fact that it has had to completely transform itself in modern times. Poetry has had to reshape itself in order to respond to the death and destruction of the twentieth century. After tens of millions of people die in back to back World Wars, it gets pretty hard to talk about roses and love.