I have a 6600. the machine won't sew with the presser foot up so that isn't the problem. Usually when I get nests in the machine I've found what Paper princess & Watson suggests is the problem: ' Sometimes my thread jumps out of the arm that goes up and down and then I get a mess, so check that your machine is threaded correctly." I've found this happens more on the 6600 than my other machines. From Watson '' Also, at one point, when I first got mine, , I was putting the thread where it is supposed to go when I fill a bobbin, instead of where it is supposed to go when I sew.' So try that too.
I find the 6600 not very fussy about which threads (and size needles) you use. I always use a topstitch needle. 80, 90, very rarely 100 to FMQ on this machine.
Is your sample the same fabrics and batting your quilt is? If it isn't, you should consider making up a (not tiny) sample with fabric the same or as similar to your fabric as possible. If its working on the sample and not on your quilt the problem is probably drag on the quilt.
This has been by far the easiest machine I own as far as tension goes. When I find a problem it has always been operator (me) error.
When I have tension problems I:
1. Rethread the machine (nests on back are fixed by this usually) I do this every time and it fixes 90% of the problems.
2. Rethread the bobbin
3. Change the upper tension
I have two bobbin casess, second one for FMQ I don't think I needed to buy that second one. Adjusting the upper tension always seems to be sufficient.
4. Take off the plate and clean out the bobbin area with a pipe cleaner and oil machine.
5. Change the needle on the machine to a new one if thread is breaking.
Here is a great handout from Cindy Needham/ Superior Threads about adjusting the tension setting. However this assumes that your machine and bobbin are threaded correctly.
https://vw-superiorthreads.storage.g...NSION-2012.pdf
Superior Threads has a big education page
http://www.superiorthreads.com/education/
Here are more hints from Cindy Needham
https://www.youtube.com/watch?v=4tsUgjsFpTo
If you were close by, I'd have you come over and we could sew together. If all else fails bring the machine in to a sewing machine repair place and have them help you. Good luck