NOTE: AMD GFX cards are not supported for hardware video decoding. Select "Decoding Method: Software" under Settings -> System -> Video -> Acceleration and enable "Allow multi threaded software decoding"
Switch to the desktop and open a terminal.
deb http://mirrors.xbmc.org/apt/steamos alchemist main
wget -O - http://email@example.com | sudo apt-key add -
optional: APT pinning
Pin: origin mirrors.xbmc.org
Pin: release l=SteamOS
sudo apt-get update
sudo apt-get install xbmc
NOTE: When Pulseaudio is configured to be multi-channel output, e.g. 5.1 profile from within pavucontrol, 2.0 streams are automatically upmixed, no matter what setting you have chosen in xbmc. This is default pulseaudio server behaviour. You can workaround that by disabling enable-remixing in /etc/pulse/daemon.conf
enable-remixing = no