Comment 5 for bug 1594024

I have the same issue. Headphone output is too low.
There is a warning when you press Vol. up. After confirm this warning with "OK" its inessential louder.

You can disable this warning in Terminal with

gsettings set com.canonical.indicator.sound warning-volume-enabled false.

But it has no effect on loudness.

You can start alsamixer in Terminal with

alsamixer -c 1

In my case, i cant change Master. Its at a level of 67.

I tried in Terminal

gsettings set com.ubuntu.sound allow-amplified-volume true

and i thought its a very little louder. But not as loud as it should be.