What macddmac proposed in Post#4 is exactly what is happening.
The RCA out on the Marantz is most likely variable level (the manual does not state this, but, it does not explicitly say Line Out either). The Onkyo has a Line In (aka. it is expected to be a fixed level signal). As you increase the volume (via the Apple device or physically on the Marantz) it will eventually clip the output.
The way to use your setup safely would be to not use the Marantz or Apple device for volume control. Instead, set the volume at a level that it is not going into protection and then use the Onkyo for volume control.
If you must use the Marantz for volume control, the only way around this is to Max out the volume on the Onkyo and hope the Marantz signal level does not max out before the Onkyo is sounding like crap from playing louder than it is designed. Hopefully, you will never need to put the Marantz volume at a level that causes it to go into protection.
That said, now your Onkyo is at risk of being blown and the Marantz might still to into protection if some party guest does not know this trick. Double trouble.
With regards the music coming from the Marantz with nothing connected...

.
Just curious, why the kluge? Why not connect the Apple device directly to the Onkyo? It has sufficient wireless capabilities to make this a breeze.