[ADTech]

I also missed that your 8-way splitter was actually a distribution amp. Your tests with the SNR readings indicate poor incoming signal strength of the UHF signals rather than insertion loss from your distribution system.

Your problem channels are all UHF and come from either San Miguel or Tijuana. Unfortunately, both signal paths are obstructed by 200' hills about 3/4 of a mile away. These obstacles will significantly attenuate and potentially scatter the incoming signals.

It appears that your tuner's software is displaying the SNR of the incoming signals. Bare minimum should be around 16 dB for a perfect signal, 18-25 for a poor quality (multi-path degraded).

Channels 8 & 10 transmit from La Jolla. You can probably see their towers at night. They will likely be easily receivable off the back of any UHF antenna without effort. We just need to make sure you don't accidentally turn them into a problem.

Several things to think about:

If your existing mount permits it, tilt the front of antenna upwards a bit (5-10°). This is easy to do with a J-mount, but won't be doable if you have a fixed vertical mast

The UHF-only pre-amp has good potential to help as long as the problem isn't multi-path. Amps won't fix multi-path.

Do consider the upgrade to the 4-bay if the UHF-only preamp doesn't solve the problem. It "flattens" the reception window in the vertical axis without affecting horizontal beam width. We'd like to keep our horizontal beam width (due to the spread between transmitter locations) unless we are suffering from multi-path. In that case we need a more directional antenna and will likely have to compromise.

The 8-bay antenna will be a much narrower beam-width than the 4-bay. It will be much more directional than the 2 or 4-bay, which will help in rejecting reflected signals (multi-path), but may have the undesirable side-effect of becoming too directional to pick up both Tijuana and San Miguel signals simultaneously.

Bottom line is you'll have to try a few things until you get it right.