Rather than use kmixer, use the CLI bits for alsa. Open up a term via ssh or locally with 'CTRL' 'ALT' "F2" (get back by doing 'CTRL' 'ALT' 'F7')
Will give you the sliders to set your outputs. Once happy, hit 'ESC' key to exit application. Then to save it do a:
This will make your settings stick on a reboot. Keep in mind, that if you use the "Rebuild Image" option from the web admin, your settings will have changed again.