Someone found a solution here by changing Windows to 1-channel. (I’m pretty sure you need to plug-into the left channel.)
This -6dB “problem” is a “feature”. When you record in mono with a stereo interface the signals are cut in half (-6dB) so you don’t get clipping when both channels are mixed (summed) together.
That always works! Or you can simply boost the with Amplify effect after recording. (But it’s probably better to kill the “silent” channel because it may have some slight noise.)
You always need to leave some headroom anyway because the peaks aren’t exactly predictable and you need to avoid clipping and you’ll always need to adjust the levels for an audiobook.
Low digital levels aren’t a problem. Pros often record at around -12 to -18dB. (You don’t need to leave that much headroom.) Sometimes low analog levels can be a problem or an indication of a problem, but it doesn’t hurt to turn-down the knob.
There is a link to the Audiobook Mastering Macro here. It will nail your peak and RMS levels ever time!