The report you have posted strongly suggests that an amplifier should not be needed, certainly not one with over 20 dB of gain. That leads me to be suspicious of the condition of the old coax. As a test, I would substitute a known good coax, running directly from the antenna to the tuner, bypassing the amplifier. Wet, moving tree limbs can cause reception problems but so can old wet coax. If you take a connection apart, look carefully for signs of moisture and corrosion. If you find either, replace the coax and the part it was connected to.
If you are near a Home Depot, their return policy is incredibly generous, which means you could purchase a 100' cable with connectors, while taking little risk.
I'm not sure if I found the antenna you listed, one web site gave a description of a fairly large UHF / High VHF antenna. If your existing antenna has no UHF section, you would still receive quite a few stations, but not as reliably. If I were to recommend a new or replacement antenna in your situation, I would suggest an Antennacraft HBU-22 or similar UHF + high VHF antenna.
http://www.solidsignal.com/pview.asp...0Antennas&sku= That is because all but three of the digital stations are transmitting on UHF "real" channels. KGW, KOPB and KPTV transmit on channel 8, 10 & 12 respectively. The type of antenna I referred to, includes elements designed to cover all of the UHF channels and VHF channels 7 through 13.
Also, if I were to making a recommendation for a new install, I would not suggest an amplified splitter such as the CM-3043. Instead I would recommend a simple passive splitter. An antenna such as the HBU-22 should receive enough signal to drive one to four receivers using the appropriate passive splitter.