Does the chiro do the c1 c2 /upper cervical adjusting also?
If no,t that might be one more thing to try, from an expert.

If neck muscles are tight - even if they don't feel painful they could still be tight- does he use ultrasound , NIMMO, IF stim, low level laser, any mention of trigger points?
*trigger points can cause referred pain like headaches and many other referred pain spots.
clickable chart for trigger point info -
http://www.pressurepointer.com/pain_reference_chart.htm