Using 64-bit ('aarch64') Arch Linux ARM on a Raspberry Pi 4 with 8 GB of RAM presents several challenges related to headless (no HDMI) boot, USB support, and a wireless network adapter that sometimes fails to initiate. This short guide can help in overcoming these challenges, while also providing more background information.
Some network supporting Denon AV receivers have functionality that allows them to interact with other devices (such as 'trigger out' ports and RS-232C support). Cheaper AVR models sometimes miss these. In this guide, a Raspberry Pi is used to extend the functionality of a Denon AVR-X2700H, to allow the AVR to turn other devices on and off based on certain events (power on/off, switched input, etc.). My use case involves an active subwoofer that lacks an energy saving (standby) mode.