I use Connecting Threads thread in my 6600, with no problem. I do have the special bobbin case, and it is well worth every penny. I got it from the dealer. In addition to what others have mentioned, I set the machine to Mode 2 - Pattern 11, set the foot pressure to "1", and set the speed to medium. I also use a Supreme Slider and gloves. I agree that you may be going too fast if the thread is breaking. Good luck and I hope you are able to resolve the issues. The 6600 is a wonderful machine.
I am sorry about the loss of your son. My heart goes out to you. I lost my son 6 years ago, and miss him everyday.