Just looked up the spec's on your Onkyo. Here's the way I interpret it:
To me it looks like the 90W out at 8ohms is taken by a compromise between voltage and current.
To get that power into 8 ohms the amp's supply voltage has dropped significantly and at this point it's literally out of juice.
The headroom quoted 120W at 8 ohms gives me ~31Vrms while the 90W figure is at 26.8 ...
To me it looks like most of the specs quoted here are mis-leading.
Perhaps all the receivers today do this. Just that this one gives me enough info to understand what's happening
Personally, I'd dump this receiver ...
