I figured out how to correctly adjust the tension while quilting with my Bernina 830 - the bobbin thread needs to be threaded through a hole in the arm of the bobbin casing to adjust the bottom tension. I tried this and it works much, MUCH better. So the problem was not with my sewing machine, but instead my lack of knowledge. I would still recommend getting a Bernina 830 over any other model of sewing machine. Happy sewing everybody!