Having found the telnet access on my Denon AVR 6500h, I've built out all the UI the poor little screen and remote control wished it could deliver. Clearly they prepared for amazing features and tools, but I guess only for the dream board or the fancy home integration companies. It's staggering how far I can push the thing and how little is surfaced on the UI.
Denon actually tend to play nicely with local control, with telnet, RS-232 serial, and local network APIs that are easy to use. My Denon receiver worked OOTB with Home Assistant.