How can I increase the bass in audio (documentary narration, people speaking)?
An article by Larry Jordan gives the following idea for using the Equalizer effect in FCPX:


  To warm a voice, here are the settings I recommend you start with:
  Men: Add 3-5 dB around 180 – 200 Hz.


There's more info in the article.  Based on this, I was able to make several improvements to the audio in Resolve.