I don't know that there are any TENS type professionals here, you might ask on a Physical Therapy related site for best TENS settings.

Is your neck having soft tissue pain or is it something due to discs or spinal cord bulges/impingements.

I have an IF stim that is very nice for muscle pain - digital so no accidental dial changes that zing you. I was trying to figure out my dad’s TENS for his low back pain but my brother had borrowed it and changed settings , dad doesn't recall what they were before..
Then the battery went dead and it doesn’t have AC adapter like my IF stim does..

Have you been checked for possible trigger points?
Those can cause referred pain and symptoms like headaches and various muscle pain.
http://www.pressurepointer.com/pain_reference_chart.htm