Panasonic DP-UB9000 HD Playback Help

[thxthx]

It seems with the lack of UHD players being manufactured that Panasonic have hit the high end of the market with their DP-UB9000 player, which I recently purchased. It's a solid unit! Unfortunately I am yet to upgrade to a 4K display, but I'd like to enjoy the Dolby Atmos tracks found on most 4K discs (not always on Blu-ray).

Straight out of the box I was able to play 4K discs through my HD display, but they appeared rather dark. After playing a few different 4K discs, I played the Blu-ray versions to compare the brightness. The Blu-ray versions were fine. However, after having played Blu-ray discs, the player now won't play the 4K discs anymore.

Is anyone familiar with this unit and could advise what settings I need to adjust to enable me to playback 4K discs through a HD display (JVC DLA HD950), and how to get the correct brightness level for 4K disc playback?