Do you have the Sweet Sixteen sit down (move the quilt) or the Simply Sixteen frame (move the machine)?
I have the HQ Sweet Sixteen sit down. I use a wide variety of threads, from 12 wt to 100 wt, cotton, silk poly, metallics. They all work fine. Every time I change my bobbin I check the bobbin tension and adjust as needed. Then I can fine tune the top tension. HQ has several good youtube videos that show how to do this.
Rob