It is simply the issue of undestanding input and output voltages of devices.
As you've described, the 2x4HD is set for 2V max input. So you need to make sure the source to that (Denon in this case) is not outputting more than 2V - or use the 4V setting.
Then you need to understand if a device has internal gain (if it amplifies or reduces the voltage from input to output). The 2x4 HD can do both depending on the settings. Worse - if you input the full 2V to the 2x4 HD, and then the filters inside actually increase that further in the digital domain, you can get digital clipping (when the result of the DSP is greater than 0dB FS).
And then what output voltage the 2x4 will output from the input voltage, any gain and filtering - and what the next device in the chain (Fosi) can accept (or needs).
Gain staging is about understanding the gain of each stage in the system (each device), and ensuring that it is all set up to work correctly.