Many older amplifiers support wired remote control through connector jacks or ports. My 10-year old Marantz also supports this function, using a simple RCA connector.
Originally these were used with external IR receivers when the unit was not kept in plain sight.
I wonder if there is a way to control the amplifier using an Arduino, which probably means by generating the required signal over the connector instead of receiving it from an IR receiver. Any pointer to a similar project is appreciated as well.