I've installed the official KS plugin, activated it in Playback and restarted. However, moving the windows volume meter makes the music volume change, leading me to believe that nothing's happening. I'm really not sure what the problem is, so any help is appreciated.

You should be glad that you were able to hear something at all I also tried this plugin. (I like the idea of kernel streaming)
When i started Playback, the computer switched off like he would when you'd plug it out. Total crash... Black screen and no power.. what a "nice" surprise.. especially in memory of all my unsafed files..

I think it has something to do with my soundcard (ATI 9000 series).

It was quite annoying, cause i had "Continue Playback after restart" enabled, and having foobar on windows autostart-list is really crappy in this case.

anyway, since then i never use components which have something like
in their description

I think (but I'm not on windows anymore) that the main volume slider still works when KS is used, it just bypasses the mixer. You can test if the windows mixer is still being used by trying to play back audio in another app while fb2k is playing. The main volume slider may be linked to a hardware control.

Hmm, okay. Slightly offtopic, but I assume the volume slider should always be at 100%? Just the basic slider.

Not if you don't want to blow your eardrums

If you're looking for bitperfect digital out or something like that, I think it should be 100%. Or at least here it is 100%, but it may depend on the sound card?