Hi Adam - welcome to TS!
First off, you have probably found that the menu structure is pretty confusing, as is the manual to some extent. For a quick reference on how to adjust everything, take a look at the
DR-70D FAQ.
Did you omit a preamp from your gear list? The NTG2 is a mono mic and has an XLR output, so I'm assuming your preamp has a stereo 3.5mm output? I'm confused by why you would be using that cable otherwise. Without knowing that or your settings on the 70D it's hard to pin down your low level issues.
I also think that paragraph you quoted, while true in theory, is not as true as it once was. You do
not need a preamp for this setup. The 70D should be fine on its own for what you're doing unless the subject you are recording is quite a long ways away. Here's what I'd do:
Ditch the battery in the mic, run an XLR cable straight to the 70D, and enable phantom power. For dialogue, I'd start with MID level gain range, but you might need to go up to HIGH. Avoid HIGH+, as that is where the 70D preamps do finally start to get noisy. See the FAQ if you don't know how to change these settings.
Good luck!