I'd say it's more from the pushing on the sides of my head, not so much from the volume of the music. But you are right, and I have been thinking about stopping listening to music trough headphones when working, and use speakers.
> If you're sitting with headphones on for six hours, maybe you should get up a few times.
I have to say that my current working desk and rhythm is pretty much perfect as far as I know. My eyes never get tired, the screens are a good distance away from me, and I don't have to look down. I have a decent computer chair, and I take breaks about every hour, to go outside or get a new cup of coffee (but I do that with my headphones/earbuds in).
Heck, no employee can offer a better working situation if you ask me, I'm all for the working remote - office not required-idea! :P
Quiet is better than loud. The more sound isolation your phones provide from the outside world, the quieter you can play your music and enjoy it.
If you're sitting with headphones on for six hours, maybe you should get up a few times.