Comment 37 for bug 893210

https://bugs.freedesktop.org/attachment.cgi?id=58415

0:72 = 0008
0:74 = 1001
0:76 = 2050

if you look at the functional block diagram, the headphone path and line out path have only two volume controls

pcm + headphone

and

pcm + master

pulseaudio just keep PCM at 0dB as Element PCM is defined in analog-output.conf.common

so the effective dB range of headphone is -46.5 dB to 0 dB