FWIW:
To me growl is a wiered thing that I've never been able to put a finger on the source so I'm guessing it's a combination of factors that becomes more pronounced when they are aligned, or just something that results from something I don't do - like phase switching/whatever. There's definetly a growl that comes from strings/setup. The growl I experience the majority of the time comes from the strings grazing the frets on slides. Some pups will pickup that sound better than others. Some pups seem to have a bit of growl otherwise.
I don't recall any pup I had that had any significant growl other than slide. I had a pup that was so burpy once that was basically the only sound it had. Never had that in a growl form.
To me growl is a lot easier to recognize when it's present than it is to generate.