Rondolet- There was another huge thread about the 6600 and FMQ issues just the other day. Check it out:
http://www.quiltingboard.com/t-154486-1.htm
I had the same problem with shredding and some of the Glow in the Dark thread. I switched to a metallic needle ( like I use when I use monofiliment thread) . I got the thread to stop shredding but never was able to get the tension back to where I needed it to be. I've been doing my own FMQ for 8 years so I know a lot of the basic tricks and I still wasted three nights on this tempermental machine. ARGH! I'm going to raise the feed dogs but right now I'm so turned off on quilting with this machine that it's not worth it to sit down and try. Never quilt angry--they say. Good luck.