Welcome, cycomiko
Keep in mind that the tvfool report is only a computer simulation, and is known to be less accurate for 2Edge and Tropo signals than for LOS signals.
If you look at the terrain profiles for ABC and CBS from DC you will see that there aren't any direct signals from them because of terrain interference. The signals arrive at your location with the benefit of bending from atmospheric refraction and diffraction as they pass over the peaks.
The transmitter is at the left; your location is at the right.
WJLA
http://www.tvfool.com/?option=com_wr...ALLTV%26n%3d13
WUSA
http://www.tvfool.com/?option=com_wr...ALLTV%26n%3d13
In addition to the terrain interference, signals 82.9 miles away are blocked by the curvature of the earth. Reliable reception of signals that far away is not to be expected.
Your 7777 preamp is probably being overloaded by your strong local signals, which makes reception of weak signals more difficult. CM cautions against its use in strong signal areas.
http://www.channelmaster.com/TV_Ante..._p/cm-7777.htm