Why does my mic produce low volume in FCP?

In the Record Voiceover pane, switch the input source to one of the mono channels from your external box and test:

since it looks like you are using a mono microphone. There maybe stereo phase cancellation.

MtD

"Red" is a bit subjective. I'd go by dB's instead. Different companies have different ideas of what "red" actually indicates or means.

There was a problem with some Blue Snowball mics a while back I remember. It was some issue with their firmware and FCP. We'd always have to bring our own Sennheisers to clients instead of the mics they used, just in case.

What mic are you using specifically?

Does it require phantom power?

Have you tried changing Mic Input levels in System Settings? What are the levels like there when testing?

This is interesting.

In my testing I see the same thing; I can type in any value but it reverts to some other number as maximum.

The input gain slider in FCP is directly linked (and will actually adjust) the "Input Volume" slider in System Settings > Sound > Input, and vice-versa. So Input Gain in FCP's Voiceover window is the same setting as System Setting's Sound > Input Volume.

Interestingly, System Settings > Sound > Input Volume will remember different values for different mics.

I also noticed that different mics I tested have different max values for input gain. My main VO mic's max gain is 21. Unfortunately, none of my pro mics have any "volume" adjustment so I can't test that.

It's as if FCP or macOS is reading some spec from the mics themselves and knows that mic's max dB input value or reading it's output impedance or something.

Record voiceovers in Final Cut Pro for Mac - Apple Support

There there's an interesting line in the manual: "Note: If the Input Gain slider is dimmed for a selected input device, try adjusting the gain on the device itself or its software utility (if available)."

So, I'm not sure. Like you mentioned, FCP input (gain) is much cleaner than what I saw in Resolve and it's more in line with field recording and dual system sound I've recorded on sets. Perhaps it's the opposite and iMovie and Resolve record Voiceovers too loudly (and possibly more noisy).

None of my mics dim the input gain slider in FCP. Changing from Stereo to Mono doesn't affect the max Input Gain value.

I can't test right now, but can you type in a higher gain value?

I don't think so. Elsewhere in FCP if you can raise a value higher that the slider allows, you can click in the value box and use the mouse track wheel to increase it. This doesn't work in that box. That would seem to indicate that the highest value allowed is 12.