I have a Cambridge Audio amplifier, which is too old to have HDMI or Ethernet control. Luckily, these amplifiers include a "control" input which accepts demodulated infra-red signals. This port was provided so that you could connect all your hifi equipment together and use one remote control for it all.
I've written a few iterations of software for Raspberry Pi & Arduino to control the amplifier.
v1 - Pi control using command line
v2 - Arduino control auto on/off by detecting audio signals
v3 - Pi control via HDMI - now my TV remote seamlessly controls my sound system