When you say "most channels", are these SD or HD ones? SD channels on a UHD screen will struggle.
Setting your V6 to output at 2160 only, will mean it does all the upscaling regardless of the source. Setting 1080 only will mean your V6 up-scales SD to HD, your TV does the rest.
Enabling 1080 & 2160 is often a good compromise, but you may need to experiment.
If you still have problems, then specific channel names and the make/model of your TV will be useful.
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