It appears to adjust the loudness with volume.

Plugged in at home I have the volume at 0dB and if you adjust the loudness it makes no change to the output. Where as if you crank the volume down (and adjust with the external amp) changing the loudness does have the desired effect.

I didn't realise this until I played with it. Damm cool if you ask me! Here I was worried about tweaking the loudness up in car.