That's fantastic.... happy to help... I know how frustrating things like this can be so am always happy to share any hints/tips/avoids I have picked up along the way. Happy listening!!
I have the same issue... the native apps on the TV will USUALLY be better than using the apps on another device (V6, Apple TV, Firestick etc.). In theory, Prime should be output at 4K and HDR from the V6 provided you have enabled 2160 in your video s...
You should definitely be able to get it to work then... that's a decent av receiver! Hopefully, your HDMI 2.0 cable will work. I didn't use the one supplied by Virgin as it didn't work properly for me either. So I used 1x HDMI 2.0 from V6 to Receiver...
Ahhh..... okay... looks like your receiver may not be able to pass through 4K video signals. Is it an older model? If so, that might be the issue. If it's a newer model, then have a look at the specs for the receiver and check that it can handle 4K.....