In order to determine the advantage of stacked antennas you should add 2.5 db to the gain of a single antenna. But even then the spacing must be optimum, which can't be true on all channels at once. Next you SUBTRACT the 3 db noise figure of the 7777 preamp. The gain of the preamp can overcome the loss of the coax and splitters, but never add to the NM.
As for the low power stations, most of them are on low band VHF. Perhaps your stacked yagis are not intended to receive channels 2-6.
Next, verify your location using the online maps scheme.
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