On the guides I found, there's two main ways to set environment variables: system-wide in /etc/environment oder user-specific in .profile & related files.
The former does not recognize reference to the home-folder and in general isn't the proper location since this is a setting for myself. However, .profile is only read by shells as it seems, and not even that: I tried to set KDEHOME in my ~/.profile, logged out and back in, opened bash, and it was not set. Amarok also ignored it.
So, where should I set it?

After a lot of experimentation, the only way that worked for me was putting it into .xsessionrc - oof