I agree that your problem is likely to be FM interference. Unfortunately, the frequency of the strongest station is 89.1 MHz, and almost all FM traps pass 89.1 with little attenuation. This is because channel 6 is on 82-88 MHz, the trap should not interfere with channel 6 reception, and the trap can't be designed with rejection that close.
A better solution for you is to install a HLSJ between your antenna and the preamp, and put a 75 ohm termination on the L port of the splitter/joiner.
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