Have you swapped your HDMI lead and/or TV port to rule that out?
Are there any specific channels you think are poor?
I wouldn't recommend having all the outputs enabled in your video settings - 1080p and (if you have a UHD TV) 2160 are probably the 2 that you need. Giving the box the option of outputting a lower resolution does run the risk of it doing just that.
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The coaxial cable is either RG6 or the thinner RG59. In a typical home the length of the cabling does not make the disadvantage of RG59 (greater attenuation) a problem. As streamed videos are "pin sharp" I see no obvious problem with the cable itself.