I have a Pfaff Creative 2170 and a Pfaff Performance 2058. There is no dial or knob to change the bobbin tension. The only way to do that is by tightening the tiny screw on the bobbin case. I keep two bobbin cases - one has a "scratch" mark on it - and that is the ONLY one I mess with while the other one I never mess with.
Interesting note ... Pfaff says to always place the bobbin when winding so that the word "Pfaff" is up. Another thing that may impact the tension is the foot if you are doing zigzag or decorative stitches - always use a 2A or 1A (these feet have a raised ares underneath). Also, I always use some sort of stabilizer as well when I do decorative stitching.