Do you think you have to be a tortured soul in order to be creative? A study done at the University of Toronto suggests otherwise. It concluded, "volunteers were better able to solve challenging word puzzles after thinking happy thoughts and listening to upbeat music." On the other hand, the researcher, Amanda MacMillan concluded, negative thoughts limit your ability to think outside the box.
Of course, it's possible this applies only to Canadians... Seriously, there is a flaw in applying this too broadly. There's a big difference between solving word puzzles and coming up with a storyline, for example. I believe that all kinds of negative thoughts and emotions can lead to great storylines--any number of people have used writing as a way to create a world that is different from their own unsatisfactory reality.
However, I believe that music IS a good way to stimulate creativity--but whether it should be upbeat or downbeat depends on the individual, his or her mood, and what he or she wants to write.