Yomaxx - it looks like you have the Comcast 4K Box - XG1v4. I never upgraded because there is zero content (OK - maybe past Olympics) that I know of in native 4K from regular broadcasts. Maybe some other folks can chime in that have the newer 4K Comcast STB's as well.
OK - make sure of the following:
1. Ensure you have enabled "Enhanced Format" which will correspond to HDMI Inputs 2 & 3 on your Sony 900F.
To play back content in 4K, connect the TV to an HDMI device which supports 4K with a High Speed HDMI cable.
To view 4K picture with enhanced format (18Gbps)
*, follow the steps below.
*4K (50, 60 Hz) of YCbCr 4:4:4, YCbCr 4:2:2, YCbCr 4:2:0 / 10bit, RGB, etc.
- Prepare the Premium High Speed HDMI cable which supports 18Gbps.
- Connect the HDMI cable to compatible HDMI terminal on the TV.
NOTE: Compatible HDMI terminal depends on the TV model. For the details, you can refer to the FAQ below.
Which HDMI terminals do support enhanced format (18Gbps)?
- Set HDMI signal format to Enhanced format.
- Press the HOME button on the remote control.
- Select Settings.
- Select External inputs in the TV category.
- Select HDMI signal format and set to Enhanced format.
2. Try setting your Comcast box to 720p or 1080i resolution. Again, folks with the newer Comcast 4K STB's can chime in here, but I would not let Comcast upscale anything.
3. New "high speed HDMI cables" will make a difference - - especially if you have really old HDMI cables.
4. For Amazon Prime - - you should be able to see when you select the program whether it's 1080p, 4K and/or 4K/HDR. After starting your program - - just pause it with the remote and read the information below. Also - - do not stream from Comcast. Stream from the native apps or a separate streaming player. (Unless you have a very slow internet speed which you should be able to check via your computer. If you can get 20 mbps down - - you will be fine with Netflix and 4K/HDR content. 1080p content consumes around 5 MB downstream but looks fantastic with the Sony 900F upscaling.)
5. When you get your new Denon X3500H - - it should all work, switching wise, since it's HDCP 2.2 compliant. But again - - do not let anything "upscale" any content except for your Sony 900F. That includes your new Denon AVR.
Let's start with that and see if you can tell a difference. Hope this helps.