I would say that swapping to the CRT doesn't narrow it down to the receiver entirely - it's still possible that the Wii itself is putting out an odd signal that it can't do anything with. Best to connect directly to a TV and see what it does. Feeding audio back in as mentioned has no real downside on the Wii since it doesn't use any audio formats that aren't allowed to do that (as would be the case for Blu-ray, for instance).
However, the typical things to try would be to check for a firmware update for the receiver (it has Ethernet at least), soft resetting it (unplug for 10+ minutes) or resetting the microprocessor (which will wipe other settings too) - to do that, turn it off via the power button, then hold down preset 2 and 3 and turn on via the power button. Release the preset buttons when the screen starts flashing about once a second. Keeping in mind that the core of more modern receivers is pretty much a little computer, so resetting it can end up being necessary.
Could also try to look through the owners thread for that series here.