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You do not have a filter problem..
Fuzzy picture on low channel numbers is ALWAYS a low signal problem..
Either you need to get the cable company to boost your input, run a new higher diameter cable for you to supply your facility.
You may need to get better quality multiplexers (amplified splitters) with a higher gain..
You do not have a problem with the direcTV because it is on a Higher channel frequency and the signal is stronger.