I'm not worried about overloading the preamp with the TV signal levels shown on your report. I suspect that the Radio Shack amplifier is noisy... notice how RS does not advertise the noise figure for the product:
http://www.radioshack.com/product/in...tent=CT2032189 The Antennas Direct PA-18 has a very low NF (less than 2 dB) and enough gain to drive the cable and splitters you're describing.
But, reliable reception starts at the antenna... The DB8 is a viable option, that you're not seeing much UHF reliably suggests to me that you need to go higher and/or switch to an even higher gain antenna such as the Antennas Direct 91XG or Winegard HD9095P. For VHF, I would also make a similar recommendation, higher and/or lager. An Antennacraft Y10713 or Winegard YA1713 comes to mind. You're facing 'deep fringe' conditions that call for the largest antennas and a low noise amplifier.
I have several HDHR tuners, they are not the least sensitive tuners in the house, but they're not the most sensitive either. I would be curious to know what your HDHR tuner sees when connected directly to one antenna, with no preamp, just a short coax, an extension cord for power and a long CAT-5 cable (for testing purposes... the HDHR would be outside, just below the antenna.)
The bottom line is this, until you have a reliable signal at the antenna, no amplifier will be able to help you. Amplifiers can't create a reliable signal if the antenna is not receiving one. Amplifiers are only able to 'push' signal through cable and splitter losses, they can not 'pull' signal from an antenna or out of the air.