The Antennas Direct 91XG was not rescaled for the 14-51 UHF band, but the DB8E was rescaled to improve the gain at the low end.
The 91XG and the HDB91X yagis have a tilt feature built in, but the DB8E does not; it would be a custom mod. I have received several reports from people who were located behind a hill and have used the tilt feature; they were able to receive channels that were impossible before tilting the antenna.
The only way you can do better with an antenna is with a custom designed yagi for channel 15.
It is impossible to receive signals weaker than NM -15 dB because they are buried in the Thermal Noise Floor below -106 dBm. If you are able to receive channels listed below -15 dB NM, either the tvfool report is wrong or the signal has been enhanced by Tropospheric Propagation.
Here is the System Noise Figure calculation for the above diagram:
You can do a little better if you use a preamp with a lower noise figure and a lower loss balun:
Yes, the KT-200 would also be my choice. Keep in mind that the ultra low noise preamps are more sensitive to static damage; that's the trade-off. The Antennas Direct PA-18 might be a good compromise design for your location, but you haven't told us which CM preamp you are using.
I second the suggestion to use the best tuner you can find; they do vary in sensitivity.