We moved ours from 555-579 down to 255-279 and works good. before you decide maybe connect your spectrum analyzer up to an antenna and see waht you pick up. If you see any weird transmissions then assign your Qam's around that stuff. I would not put a QAM carrier where there is an off air channel for example. I know CATV is a closed system but why make things harder on your self. And YES if you are doing leakage then the system should be tight. But there is always the "handyman" factor. Some homeowner who runs their own crap after you leave.
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