Welcome to the QB from SE Michigan first of all! I have the Brothers 1500se. They are similar machines. I have been having trouble with the tension on my machine and have a similar problem right now. So for right now I have stopped trying to do anything. I even had t he machine services and use only Brother's parts. It is quite the quandary. One other problem I discovered was that my frame was not level and that makes a big difference. With all the things you have tried my next call would be to their tech support service. They were very helpful to me even though now I know it is in my setup.
Why it is not going for the beginning could have more to do with the " compatible " parts. I would try the ones made by Brothers only because it could be just a tiny bit difference and this could make the difference.
Again welcome to the QB and please just jump right in.