The problem is that the V6 box tries to pick the highest available resolution. It tries to negotiate with the AV Amp and picks 4k, which has HDCP 2.0 protection. You AV amp then connects to your TV which is not 4k and cannot get past the HDCP protection and you get a black screen.
We should be able to specify on the V6 box which resolution we want to use.
The fix for me is to get a splitter for the HDMI output which is not capable of 4K. This plugs into the V6 box and sends the signal to the AV amp and then the TV as 1080p. You can get them off eBay for < £20.