Hi....I also have the 6600 which I bought second hand last year.
To begin with I had real problems with tension but only when FMQ.... and I think that part of the problem was that I was threading it and then sewing without first raising the needle to the up position. I read somewhere that you need to do this to reset the tension....the other possibility could have been that when I took the bobbin area apart to clean it I noticed the white disc under the bobbin holder had deep groves in it ((looked like the needle had scratched it at some stage) so I also replaced it....anyway ever since it appears to have been fine, I can use all threads in it but it really isn't keen on King Tut.....but maybe I haven't learnt how to change the tension to accomodate the thread...but it is fine with all other threads......Guttermann included, however I have to agree with other members Aurifil is by far and away the way the best. I always use it is in my bobbin because it has far less lint and so the bobbin area stays much cleaner.
There are some great You Tube clip regarding threads called Thread Therapy with Dr Bob.... I thought that all threads were the same but they are not....here is a link to one of his clips....he may have some tips that might help
http://www.youtube.com/watch?v=I1jq9...feature=relmfu