Have updated to the November update? Some are having problems following the update. I have the original Diamond and have had to take mine back to the update before the September one. It is now back to stitching perfectly.
I am sorry I missed this and did not see it before. I hope you have been able to solve the problem some way or other. I have a different machine, a Ruby, and never had a problem with the tension unless I did not thread it properly or put a bobbin upside down - and that can only be done with pre-wound bobbins, the Husqvarna ones fit only one way.
For freehand stippling I get into problems with the invisible thread. The elasticity of it makes adjusting the tension a matter of trial and error for me - so that means stippling on spare fabric till I get it right, or using normal thread top and bobbin.