I finally found a winning combination for pork shoulder barbecue. From another thread, I was recommended to abandon the rub, and sprinkle a liberal amount of salt on the outside of the butt. I did so, and also turned the butt with fat cap down.
After smoking for nine hours over hickory coals, the result was fantastic. No, it was sublime. With no rub to burn and turn black, the entire shoulder was red-brown. We call that "outside brown" in North Carolina. With all the rubs I have used, both with and without sugar, the outside always turned black, which was far too bitter to eat. Without the rub and the addition of so much outside brown, the barbecue needed no sauce or rub or anything at all. It was perfect. I also noticed that this was the smokiest cue I had ever smoked. I would highly recommend ditching the rub.
If you are curious about my setup, see here:
This was the original thread that inspired me: