Before getting to the amplifer question...
We do need to recognize that the C2 has a defined reception pattern as shown on the back of the box and here:
http://www.antennasdirect.com/clears...ntalplane.html That makes the aiming direction of the loops important. Looking at your TVFool plot, it shows that you have stations "all over the map" from numerous directions. While you may find an aiming direction what makes everything happy, it will be a trial and error effort. The C2 does have a generous 70 degree beamwidth for its primary lobe, but when signals arrive at the back or sides, they will often be delivered to the tuner is a less than usable condition. Capabilities do vary between various tuners, so be prepared for some variations. Some are more sensitive to weak signals, some are more capable at correcting multi-path.
Since we don't know which direction you have the antenna aimed nor which stations are on your "must have" list, we'll defer the amplifier question. I don't see a need for a pre-amp, but a distribution amp *might* be helpful. If you can install a barrel splice in place of that amp/splitter and test that converter box, that would tell us how it responds to a passively delivered signal. Personally, I'm not optimistic that you'll be able to find a single aiming direction that works for everything UHF on your chart, especially in an attic.
Also, don't obsess about the meter readings. The threshold for success is simply reliable reception, not a maximum number. If it isn't breaking up during both mild and severe conditions, you're done.