I just finished a quilt on my 6600. I used all kinds of thread: Mettlar, Superior, unspecified metallic embroidery thread, and for the bulk of my FMQ I used Dual Duty with So Fine Superior thread in the bobbin. It was the only thread I had that matched and it was late. Never had used Dual duty before. All worked beautifully. I do have the special FMQ bobbin case. I have had that weird tension thing happen a few months back. I didn't worry about it as it only happened once or twice. Now I know how to solve it! This machine has taken any thread I've thrown at it.