Looking at pavucontrol I can configure much more when I use pulseaudio. Reproducible: Always
pavucontrol is a fairly raw interface. In KDE the settings are managed differently via categories. The Audio Setup UI is where you configure cards and profile (e.g. to enable 5.1 etc). You find this via System Settings. Kmix is just the mixer frontend to control levels. So while I won't close this, it's my opinion that you don't want kmix to provide such facilities when other UIs already exist for this (and are accessible via the kmix menu).
Actually this may be a reasonable wish, especially as the Audio Setup GUI has now been removed from System Settings.