I'm an engineer that actually "sees" sound.

What I don't do is record metronome's. Nothing to be gleaned from that. You listen to actual musical sources that's what you do. Recording them simultaneously & comparing the two. Not metronome's. It's like trying to record the signal to noise ratio of an empty room. You can't. Not really. You can certainly measure noise levels, with calibrated testing devices. Just use your microphones and you'll get to know what they sound good on and what they don't sound good on.

Use your ears. They are the closest you can get to your brain.
Ms. Remy Ann David

well yes, bad idea recording the tic-tac, my bad, I guess I'll have to try with different sources to get to know the differences, there is something I think is related:
What should be the gain for the MICs ?, 8.5 ?, 10 ?, 9 ?
Having it on 10 gives me a little noise so I guess it shouldn't be 10

Who is the guy with the signature lioke "Talking about music is like trying to taste architecture."?

So fitting right now.

Your gain should be set depending on the source, try to record without clipping. Keeping the peaks from creating audible clipping is the name of the game. Also keeping it above the noise floor.

record something you hear all the time. maybe your own voice. then record with different gain levels. you want your waveforms to never touch the upper or lower edges.