I haven't had any problems with any thread so far in my 6600. This is not to say it's not there, but I don't remember ever reading anything in my manual that said not to touch the bobbin tension. I agree with Scissor Queen in that you have to change the needle size depending on what thread you're using.
I learned many years ago how to adjust bobbin tension and it has never failed me yet. It's pretty simple, but I would suggest you should only try the following if you're comfortable in what you're doing. I'm no expert or repair person by any means, just had about 45 - 50 years of maintenance experience on my own sewing machines.
Remove the bobbin and shuttle, insert the bobbin into the shuttle. While holding the lose thread end, do a very gentle "up-down" motion. The thread should move slightly through the tension, but not "run away" through the shuttle tension. If it doesn't move at all, it's too tight...loosen the screw a very slight amount and try again until you get that very slight give on the thread with the "up-down" motion. If it moves too much, tighten the screw a very slight amount and try again.