Both sound fine to me, and I'm really not sure which is more common. I think I would be more likely to say "down lower" if I were emphasizing motion /toward/ a lower area, and "lower down" if I were emphasizing the lower /area/ itself. ("Look down lower" = "LOOK DOWN" toward an area that is "lower" / "Look lower down" = "LOOK" toward an area that is "lower down.") But it's hard to think of any examples where the difference would matter very much. I think in most cases the two are interchangeable. In the line you quoted, "now lower down" would sound equally natural.