[GroundUrMast]

Your description makes me lean toward suspecting an out-of-box failure or some sort of misreading of the installation instructions. Even if the 'new and improved' CM-7777 is less able to handle strong signals, the 20 dB attenuator should have been enough to get you into a workable range.

You are absolutely correct, a preamplifier's primary roll is to overcome the loss in cable and splitters that are connected to the amplifier output. They may also negate a portion of the internal noise of the tuner. After adding up the losses and considering the noise figure of the tuner, excess preamp gain is of no benefit.

The VU-210XR claimed 11 dBd gain in the UHF band. If true, you should be able to connect 50' of RG-6, a 4-way splitter and then another 50' of RG-6 (a total of about 14 dB loss) and end up at a tuner with a NF of 8 dB... at a Net-Noise-Margin of 15 dB when aiming at a signal listed at a NM of 26 dB in the air. (A Net-NM of 15 implies that the signal is 15 dB stronger than the minimum needed for reception. You would expect to be getting reliable reception at this and higher levels.) So that translates to all the signals down to the mid range of those highlighted in yellow on your report. (This is why I don't think a preamplifier is indicated, nor is it the solution to your problem.)

The trees are the culprit.

If you can't solve the tree problem, and want to try an amplifier, a high input unit such as the CM-7778, HDP-269 or my favorite, the CPA-19 have a better tolerance for strong signal levels. (I'm not recommending you spend the money, because I don't think any amplifier will address the real problem.)