My Sony 4K Smart TV is less than 3 years old. The last two months or so it is just not the same tv it was once. Don't get me wrong it is still good color and all but the brilliant boom is gone. I have a LG tv that was the first of the LCD tv's and it has a brighter screen now. I have tried all the picture settings besides vivid too. And brightness on every level and still looks dull. Any help from the experts would be greatly appreciated.
Did you try resetting it back to the defaults? It sound like you tried everything else.
A problem with the backlight would result in a duller picture.
They sell replacement parts and are very helpful.