So it may very well be the cabling that's holding things back.The player interpolates UHD with 4:4:4 subsampling. By default, the UBP-X1000ES sends the maximum data from disc to screen, based on the capability of the connected receiving device. For example, when playing a 4K 60p disc, if the sink (receiver) permits, then the player will send resolution at 4K 60p, Deep Colour at 12 bit and chroma at 4:4:4.
If this isn’t possible, then the amount of information is reduced in the following priority order: resolution, frame rate, HDR, Deep Colour and Colour Space. Use a high-speed 18 Gbps HDMI cable for the best possible picture quality. If you use a lesser 10.2 Gbps HDMI cable, the player could output 4K at SDR, and 8bit with 4: 2: 0 colour subsampling.
All the best,
Nonoise