If I understand you correctly, you don't care about the Master Volume you just want to be able to manipulate PCM directly without having to open the full mixer. I think I have a solution for you but keep in mind that the Master channel should be functioning where I kinda get the impression that it doesn't for you. Someone might see this and have a solution for you.
Nonetheless it's not rare for someone to want their "master" channel to actually be something else. Right-click on the KMix icon in the tray and go into "Select Master Channel..." In there you should be able to change your "master" channel to the PCM channel. And that should work.
Just to make sure this works across logins... Check your ~/.kde/share/config/kmixrc file before and after and make sure the bolded line changed. Obviously your sound system and your sound device will be different but the syntax is the same.
bill@yogi:~$ cat ~/.kde/share/config/kmixrc
EDIT: Just make sure you ran alsaconf and alsamixer, etc. Maybe that's why your master didn't work but I doubt it. In any case my presumption that your master channel should work might not be totally correct so just go through all options before you settle. In any case I hope I was able to help you.