Well, after spending even more time digging around, I found the article below. Based on that, I went digging through the settings on the Denon, and in Video->4K Signal Format I changed from Standard to Enhanced.
I have a new Denon AVR-X1600H connected to an LG OLED77CX screen. I also have a new Roku Ultra 4800.
When the Roku tries to detect the monitor, it decides that 4k UHD 60Hz doesn't work.
If either of the HDMI cables connect the Roku directly to the TV, it says 60Hz works great. If I connect...