First, you have lots of signal in the air. Likely the rabbit ears would work for all the green channels in the 228 compass heading direction IF there were no other sources of interference. Once you go inside and behind trees, the characteristics of the building and the movement/wetness of the trees makes reception mroe difficult. It is difficult to impossible to predict effects like this. Your TVFR does not account for such issues.
That said, a big antenna in your attic should work ok, if the trees are not a factor. Your best move would be to position the antenna so it is above the tree tops, or aiming between the trees.
This article
http://www.hdtvprimer.com/antennas/siting.html gives you an idea of what you are up against when aiming through trees. And when the wind blows, the local signal intensity moves around with the movement of the branches.
With your report, if you were on the roof in clear air, you could do very well with a modest antenna like the RCA ANT751. Going into the attic justifies going bigger due to the attenuation of the building materials, but increasing the size of the antenna may not be enough if you are aiming through trees.
A preamp won't help this. There is no shortage of signal in the air, and a cable run of 50' is not long.