The HEOS line of Denon/Marantz also has a HTTP API with a fully public API specification[1], which is great!

You can use the ESP wifi connection to control it if you want to and it has quite a few features (afaik you can do everything the HEOS app can do). Do other vendors also have such an open approach?

[1] https://www.marantz.com/-/media/files/documentmaster/marantz...