You would have to move to the Sony 695es to compete with the NX5 and even then, the JVC has an all glass lens that can better the 695es in resolving a good 4K source.
Despite what you saw in the demo, I believe the JVC looks better then the Sony when it comes to the small details.
If you watched The Martian up to where the captain is inside the space ship _ start looking at the fine details in the equipment, look at the vent grill holes in the Captains monitor, I'm betting that the JVC resolves that better with the 4K version of that movie.
Although Sony has never admitted to the contrast degradation problem, apparently they have taken steps to remedy this, with that said, certain reliable sources have reported the problem to still exist and then you have to prove to Sony that it's affecting your unit, that in itself is a royal PITA !
Another option, although a high maintenance one is an HTPC with MadVR for your UHD discs, although a bit involved to set up, once done, it apparently betters the Panasonics ub820 and ub9000 when it comes to HDR and video processing.
An HTPC will also allow you to split the audio and video, so you may still use your out of date receiver, that's what I'm doing now.
You will also need a UHD friendly drive for your HTPC, there is a fairly simple way to deal with that.
Here are some links to mull through and in there it gives steps to set it up with Kodi Player, although you may use what any player you want as long as it can accept external filters, like MadVR, and that's what you want.
A link to downgrade the firmware to make it UHD friendly.
Maybe a bit more information then you need right now, but there it is.