Which frequency to adjust if voices/mids give me a headache.

If the irritation seems to be brash or brassy, I agree with the 1.5k to 1.6k give or take 500 Hz. assessment. If the irritation is more shrill, I find more often than not that the offending frequency is around 3.1k. It could also be both regions.

You may want to start with a larger reduction like 6 db or greater just because it is easier to hear bigger changes and then add them back in as needed after you find the offending frequencies.

Treat your room and buy better front end equipment and amplification. Anything else is a band aid.

I've done Dirac room correction and both sets of speakers still bother me a bit.  These are not bright speakers I know.  

@wrecked Since you've done room correction the cause of the irritation may not be brightness. Higher ordered harmonics caused by electronics are perceived as the ear as harshness and brightness, and yes, can be enough so that you might get a headache.

The thing about that is the room can measure perfectly flat while this is going on. Since you already know the speakers aren't bright, if it were me, I'd be looking at the electronics. Solid state amps (in general) are well known for this (which is why tube amps are still around after 7 decades of transistors...). There are solid state electronics that aren't bright due to distortion.

So again if it were me I'd try a different amp and see if you get different results.

I think I found a decent solution for the moment. 

As I mentioned I had the Lintons and the 4.2s.  I did room correction for both.  It turns out I really like the room correction for the 4.2 applied to the Linton speakers.  Goofy, I know but I'm pretty happy right now.  I also pointed the speakers straight ahead and that helps a bit. 

It turns out I really like the room correction for the 4.2 applied to the Linton speakers.  Goofy, I know but I'm pretty happy right now

Put 4 inch absorptive GIK panels on the ipsilateral and contralateral reflection points on both side walls. If you’re sitting crack up near the back wall, treat it too.

It can give more happiness (than right now) and reduce the headaches even more!!