Couple things to unravel...
1. The review on rtings.com for this tv seem to be somewhat mixed. Not liked as much as previous Sonia Bravia sets (i'm a big Sony fan, but this one seems to be a little less liked). -
https://www.rtings.com/tv/reviews/sony/x800h
2. If you have an audio system your inputs should go through the receiver then output to the TV. Essentially the TV only has one cable coming from the audio system. I say this blindly though because I don't know all the details of your system.
3. Most picture issues tend to be from the source not necessarily the TV if it is inconsistent. If it was the TV you'd have a bad picture consistently. A lot of these new TV's require firmware updates so check if you need that done. Hook it up wired or if it has wireless do it that way. If you have X1 TV service through Xfinity, look to swap out your box for a 4k box and try some 4k content through that. The existing boxes they offered up until about 2 or so years ago was 720p/1080i and not all TV's do a good job of upscaling, especially to 4k.