I'm not trying to argue with you Mike. And no doubt consumer electronics is designed to be idiot proof.
However modern amplifiers have digital inputs (co-ax sockets in some cases). I have no idea what's inside the box, but theoretically you can be putting a digital signal into the amp, where - possibly after navigating some protection circuits - it hits a digital processor chip.
I suppose with things like I2C you might expect that both ends would be powered, and on a single board that would certainly be the case. But once you start talking about "cable runs" for I2C/SPI and how it is a 2-Wire protocol (SDA+SCL+GND) then one presumes that each end is individually powered. And therefore one end powers up before the other one.