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I tried without the preamp and got almost no channels.
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Hmm... That indicates that either you had a bad cable, bad antenna, or a "bad" attic or an outside signal obstruction that cannot be calculated.
Such an attic would be one that has any of the following: Metal roof, stone or tile roof, radiant barrier, decking with radiant barrier on the top, TechShield shingles, "shooting through a gable that has metal or cement content, or similar. For a simple layout that you've described and from your TVfool plot, there should be sufficient signal power at your address such that you could run multiple TV sets throughout the whole house without ever using an amp.
Outside obstructions would be houses or other buildings or trees in the signal path to your attic.
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moved the antenna about a foot inside the attic.
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A whole foot? Think bigger. Try EVERYWHERE in the attic. Attics are black holes for signals and you have to explore them to see if you can find a "sweet spot". Be patient and be persistent.
The next measure is the obvious one and you already know what it is.