Like TV should be 60Hz, early generation 4k products where 2160p 30hz. Your amp does not support 60HZ.

Code:
https://www.marantz.co.uk/uk/products/pages/productdetails.aspx?catid=hometheatre&subcatid=avreceiver&productid=nr1606
Analogue to HDMI scaling up to 4k 30/25/24Hz
HDMI to HDMI scaling up to 4k 30/25/24Hz