My dad was looking at the JVC and Samsung and went for the JVC because he doesn't know anything about tellys and to be honest neither do I. The picture looks really washed out as you said and quite dark. I had a fiddle with the settings, like Picture (gaming, cinema etc), contrast, brightness, but nothing had any effect whatsoever. I could also only access the picture settings when I turned the Sky Box off, so I'm wondering if there's setting within Sky I need to change?
I ended up getting the JVC in the end, and have it set to dynamic in the image settings with the contrast all the way up, brightness about half way, sharpness almost all the way, and colour about 75. Dynamic contrast is set on high. That's the setting I use for the game, and it's great. One thing to note is that those settings are custom and save depending on the output you have it on, so you could have them set for one thing, and they are at default for another source. 4k HDR from video sources like Youtube can vary in quality, but thats dependent on the video and not the TV itself. I've watched some 4k video in HDR that looks stunning on the TV, so it could be your source.
As for the menu settings only being accessible when your box is off, I found my TV acting a bit troublesome when I first turned it on and tuned it in. For some reason it kept freezing, and like you, I couldn't access them main menu. I had to go to the source menu and access it through that for some reason. It's fine now, though. After doing a soft reset by unplugging it and waiting 30 seconds, it's righted itself and hasn't had any issues since. Could be you need to update the TV's firmware online.
