I was wondering what would a producer do to make a voiceover appear to sound deeper, but without changing the pitch. Note the voiceover is already deep, but not very deep.
I can think of two things:
Using EQ to boost the bass as I’ve noticed highly bassy voiceovers sound deeper.
I’ve also found adding distortion can help make it sound more raspy.
Anything else?
Various tutorials from Mike on this topic can be found here: https://www.youtube.com/@MikeRussell/search?query=deep
I hope this sets you on the right track 
A couple of extra ideas here. Look at reducing the EQ at the higher end too and you’ll get an even deeper boomier voice. Mouth proximity to the microphone is always going to help so get up close to the recording microphone.
You coulls also try layering multiple takes of the voiceover together to create a richer, deeper sound. This can help to create the impression of a deeper voice without changing the pitch.
There’s a plugin from UAD called 'Voice Of God" that does some nifty stuff with Bass resonance. I don’t have it but the examples of it on dialogue you can find videos of are pretty cool sounding. I have a free alternative called Bark of Dog that does something similar.