With all due respect.
I believe you are thinking about this matter completely the wrong way round.
Instead of thinking about what frequencies to adjust after the voice has been recorded to obtain the desired sound.
Think about utilising the optimal microphone whose sonic signature will enable the characteristics of the voice to best convey the emotions aroused by the song.
If you have a great sounding voice to record, why not capture its performance, just exactly as great as it is, without the need for any further post recording “enhancement”?
If the voice isn’t that great, why bother to record it at all?
I would be looking to use a mic with a slow, gradual, slope off the lower frequencies from 1,000 Hz all the way to the bottom of is range.
In recordings, its easy for the lower middle range to become thick and cloying or muddy. But this area is often boosted with equalisation which makes the problem worse.
By utilising a mic that rolls off those lower frequencies, you preserve and enhance the voices clarity and conveyance of emotion and separate it sonically from other elements of the mix, that might otherwise compete for similar space.
If you click on “data and diagrams” and scroll down you will see this mic rolls off from 5 kHz all the way down.
TLM 49 - Studio Microphone (neumann.com)
Here’s another popular mic. See the effect of using its Low Cut Switch.
U 87 Ai - Studio Microphone (neumann.com)
By using the right mic of excellence and the right placement of the voice in relationship to it position in front of the mic, it should be possible to get the sound you want virtually “record ready”, sounding like the finished article.
You may want to use a little compression and reverb but it should be possible to get really close to the finished sound without adjusting the equalisation at all whilst recording, even if a touch is needed during the final mixing stage.
I’ve worked on a few projects that have involve singers with low voices, and suppose the late Johnny Cash would be one of those with the lowest sounding voice. But I would strongly encourage you to leave the equalisation flat in the first instance and focus your attention on using a mic with the type of characteristics I have described.
I think that approach will be most likely to provide you with the optimal result.
People often appear to think that boosting frequencies in a particular range is the way to obtain the sound they want, and sometimes it is.
But where lower frequencies are involved (bass instruments apart) it is often preferable to use minus equalisation (the microphone above do that for you) to preserve clarity.
Take a piano for instance that extends across a very wide compass of frequencies. If you boosted the lower end with equalisation it would begin to sound a bit like a bass, and directly compete with it in the mix.
But if you sloped off its bass frequencies from 100 Hz gradually downward, suddenly there is clarity and that hallmark zing of a genuine piano bass sound; without the cloying muddiness that accompanies boosted frequencies where such boosting just makes everything too thick, giving an unclear sound.
As the sum total of voices with their reverb and instruments add up into the mix that whole area of upper and lower mid frequencies and bass frequencies just become to saturated in plus equalisation.
One thing I learnt from Gus Dudgeon was that as most instruments, particularly if an orchestra is involved, occupy the mid-range of frequencies. Rather than boosting those areas to help bring them out, it is usually better to use minus equalisation to provide clarity. If you listen to Elton’s early hit records you will readily perceive the clarity this provided.
Furthermore, if my memory serves me correctly, it is long been virtually a standard operating procedure at Abbey Road recording studios. To send signal to the reverb chamber, applying a high pass filter at 600Hz and a low pass filter at 10kHz. This cuts out the muddiness of low middle and bass frequencies along with the top end splishy spashiness that can otherwise spoil a great recorded sound.
The returning sound from the reverb chamber is much cleaner and clearer and minus the unwanted artefacts that using effects can often add to a great recorded sound.
Try and get the sound you want from the voice at source, and capture it with the optimal mic used in the optimal manner without using any equalisation at all whilst tracking.
I learnt the above from the late Al Schmitt and it seemed to serve him well.