I sincerely hope you don’t have any hearing damage! Your comment is probably better suited to a doctor or an audiologist. I’m just a guy who writes random articles on the internet.
You might also want to get in touch with Audio-Technica support and make sure you got a genuine pair of M50X’s. Even with their slightly pushed upper mids, I’ve never heard of anyone complaining of fatigue at low listening volumes. I have an S8+ too, and I find the M50X’s comfy at around 60 percent volume without issue.
Now, I have heard of plenty of people disliking their clamping force, and that’s a totally fair complaint. They’re designed to clamp a bit tighter than normal for studio applications.
It could also be that you’re just super used to bass-heavy headphones, to the point where the moderately pushed high frequencies in the M50X are really getting to you.
But again, I’m just some dude sharing my opinions. Please go get checked out by a professional if you’re having pain!