You did a great job on your first quilt - I love your color choices and the three-dimensional look. As for the balling up of the bobbin thread, did this perhaps occur at the beginning of a stretch of sewing? That's when it always happens to me and it's because I haven't pulled taught both the top and bottom threads to keep them out of the way. The suggestion of the poster above to lay out several spools of thread to audition them is a good one.