If you increase the volume by 1 dB, then of course the sound should be 1 dB louder unless one or more of the following happens:
- the increase is not really 1 dB (it is not correctly calibrated) - unlikely with digital volume control, maybe with particularly poorly written EQs it can occur
- the system changes between the tests
- there is high non-linear distortion
- dynamic compression is applied to the signal
Regarding damping factor and distortion, here is an article with some measurements.
But that's what doesn't make sense to me. If I change a sound file by 1dB and play it on different headphones, one very sensitive and the other with low sensitivity, shouldn't the difference be more in the more sensitive one, instead of 1dB in both?
Thanks for the article, it's great for understanding this kind of thing.
