This is correct. When I say headroom I mean available travel on the potentiometer to increase volume. Obviously, running on batteries is going to decrease your voltage capability and decrease the ability to drive the headphones as loud. Basically, headroom to me is how loud you can go before you either clip or max out the amplifiers ability to increase volume.
I should be able to try a few quietly mastered tracks tonight with larger dynamic range, take a picture of the board since it's been requested, etc.