Do you have any pictures? A 'high headset' means different things to different folks. This is Mia (my Arabian mare) totally relaxed:
This is Mia in a normal riding position:
This is Mia when she is concerned about having another horse near her, but not so much as to stiffen her back..or at least not very stiff. Any higher would get her a stiff back:
I don't have any pictures of her on full alert with her back hollowed out...we're usually kind of busy at that point. Let's just say I sometimes am tempted to bite her ears at that point...
I think it is more useful to pay attention to how she is moving and balanced. How stiff is her back? Is she moving loose, or with tension? At a canter, she used to stretch way out and get very heavy on the front - to the point I was worried we might flip. I will insist on a higher head at a canter because it helps her to shift her weight back, and I'll accept any tightness in the back in exchange for a safer balance. And as her balance is getting better, she is starting to do it right without getting stiff in the back.
As a suggestion, pay attention to how YOUR balance drives her balance. Where your center of gravity is, how much weight you carry in your rump vs thighs, how freely you move with her - those all play in to how she balances. And your goals in riding will affect what balance you want from her. Dressage is very different from speed. Don't look at other horses heads, but focus on what your goals are, where your horse is at right now, and how your balance and motion can help move her in the direction - over months or even years - in the direction you want.
All just IMHO. Mia and I are obviously a work in progress. Or I hope we're making progress...