Your explanation is just stiring things up...
1, At same volume setting, higher gain = higher volume.
2, Atom has pot after gain stage to minimize noise. It's the superior way. 10kohm can't clip a opamp output.
3, Voltage gives loudness, power is only required accordingly.
1) Yes, of course. However, Kosta mentioned that the volume varied between 2 and 11 depending on gain, so the volume setting
had changed.
2) Perhaps it is superior, but the gain stage shouldn't be noisy enough for it to matter, and overload performance I think matters more than inaudible noise. That's why I would put the volume control before any gain stage.
3) No, it's power that gives the loudness, but clearly, for a fixed impedance more voltage means more power. For headphones it's even more complicated as impedance and sensitivity both vary depending on the strength of the magnets used and the number of turns on the coils, and the gauge of wire used, which also has an effect on the mass of the diaphragm. All these will affect sensitivity and so just how loud any headphone goes with any specific number of volts and so how much power is needed. I accept, however, that all one can say with any certainty is that for any individual headphone, loudness is proportional to voltage.
Hopefully, things will get unstirred now.
S.