Use the DB8 with a VHF dipole added to the mast. That will get you into the ballpark, at least, for PBS and ABC. Tip sheet:
https://dl.dropboxusercontent.com/u/...20antennas.pdf
Alternatively, use the DB8 plus the 751R on the same (or separate) mast(s). Set the RCA amp's switch to "Separate", hook the 751R to the VHF input and the DB8 to the UHF input. This will allow each antenna to contribute what it does best.
Delete the second amplifier. It's probably unnecessary and it's just asking for trouble.