It sounds like you are already aiming correctly but here is a link anyway...
http://www.kyes.com/antenna/pointing/pointing.html
The
U8000 is advertised to have 12 dB of forward gain on real channels 14 and up (that's good). But, you may be loosing all that gain or more by mounting in the attic. Also, WGTV is on real channel 8 and WXIA is on real channel 10. The virtual channel numbers are of no help when considering which antenna is needed.
The U8000 is a good choice, but it needs to be mounted outside, high and free from trees, buildings or other obstructions or it's performance will be affected. You also need a high-VHF antenna designed to receive channel 7 through 13. In you situation an Antennacraft
Y10713 would be a good choice. Again, attic mounting is going to cost you quite a bit of signal strength and quality, the H-VHF antenna also needs to have an unobstructed view to the SW in order to be successful. If you mount these antennas on the same mast, they need to have at least 4' of space between them, more would be better. When I put your report coordinates in, I can see the general neighborhood you are in, as well as experiment with higher antenna elevation. There is a clear advantage to mounting higher up. For example, the noise margin for WXIA channel 10 improves by 10 to 15 dB. You could spend hundreds or thousands of dollars on custom commercial grade antennas and not get that much gain in signal. (If I were your next door neighbor, you would see 10' of mast on top of my roof...)
Yes, a preamp is needed. You have a few powerful FM stations nearby so I would suggest an
Antennacraft 10G221. It includes an FM trap filter (which you would option 'in') and also has separate inputs for the two antennas.
It does not sound like the diplexers are a problem. If they were, you would not see any OTA signals until you totally bypassed them, connecting the antenna directly to your TV tuner or DTV converter. If you drop satellite service, you will gain a small amount of signal strength if you remove the diplexers (1 to 3 dB I would guess).
By selecting the right antennas and then mounting in the right location you can hope to get good, usable signals. Then, choosing an appropriate preamplifier will ensure that your signals can survive the losses caused by splitters, diplexers and cable.