plasma-pa cannot (or, to be fair, I cannot find it ;-) ) change the volume per channel (like left 10% louder than right) as pavucontrol or kmix can. As plasma-pa is meant to substitute kmix, this would be a regression, which makes using headphones quite unpleasant for people with hearing problems. For now I installed pavucontrol and showed the affected person how to use it. But that's merely a workaround.