All advice already given is good. Other thoughts:
Tighten your bobbin first. Set your top tension to the lowest setting, start sewing. Increase the tension gradually until you get the tension where you want it, This works well for me. Having said that, I don't use King Tut thread so my advice may be useless.
Another thought - is your bobbin case possibly broken? That happened to me once. One of the tiny screws in it had been chipped somehow. The bobbin case had to be replaced.
Another time, the bobbin itself was bent inward just a tiny bit, but it was enough to throw off the quilting.