Comment 3 for bug 253422

I forgot to mention that the "headphones" volume was not changing anything, however the main and PCM volumes were modifying the headphones and speakers volumes at the same time.

A little workaround to have only headphones working, put:
options snd-hda-intel model=6stack-dig
at the end of /etc/modprobe.d/alsa-base
then in the tool to control the volume, go in parameters and uncheck "headphones" (checking/unchecking will change what is working: speakers/headphones), that did it for me.