Use 12V trigger from Denon AVR to switch NAD C352 and Behringer A500 on and off together.
The Denon AVR has a 12V trigger output, that is high when it's on and low when it's off. Trigger is read through a voltage divider (30kΩ - 20kΩ, giving 12V -> ~4.8V) on TRIGGER_PIN.
The NAD has an external IR in, which takes Extended NEC IR commands (without a carrier wave). Just connect a 3.5mm jack from the external IR input on the NAD to IR_PIN.
The IR signal is sent with this direct NEC transmitter library.
There is no external control possible on this amplifier, so I use a Sonoff Mini relay to switch the power. Just connect a 2N2222 between S1 and S2 on the Sonoff (S1 = ground, connect to emitter. S2 = pulled up to 3.3, connect to collector. Base connects to RELAY_PIN through a 1kΩ resistor).