Instead of dropping the feed dogs, you can leave them up and set stitch length (and width) to zero. Some people prefer this saying it gives them more control. I'm not sure it would change your bobbin thread problem, though.
In addition to loosening top tension, you can tighten bottom tension. Bernina's have a hole in the bobbin case through which you can thread the bobbin thread to increase tension slightly. Otherwise you can tighten the tiny screw in the bobbin case. Some people refuse to mess with that screw because they fear they will never get it back to the right setting for regular sewing. If you understand what you are doing, though, it's not a problem.
Mary