Since everyone has said what I would have said, I would like to add that one difference is the distortion produced by the driver, a cheap 40mm driver in a random amazon headphone usually will distort a lot of the sounds. This can be caused by both the damping of the headphone, and the driver itself and how it has been designed. This would make the sound much less detailed, and dull. There are also “reflections” that a headphone designer would have to take care of, and that is not something you can fix with eq. This can be the foam in the front of the driver, or the foam on the back of the driver. Another major difference is the excursion of the driver, something like a focal clear will have a high excursion driver that would make it have a very dynamic sound, while something like a hd600 wouldn’t sound all that dynamic in comparison. In summary yes there are physical differences, just like how you could have 2 camera lenses made both of glass, one might be super distorted and mess up edges, while the other might be crystal clear.