Time to learn some music theory, I guess.

https://www.musictheory.net/

People may hate on Sausage Fattener, but I absolutely love it so that's my recommendation.

I think it's a great plugin but I miss the possibility to match the pre and post volume. Sausage Fattener is used to make things loud but you can't compare what the plugin does at the same volume.

Nevertheless, a great plugin.

Fabfilter's Saturn is fantastic for the creativity (everything can be coupled to an envelope follower or lfo) but it probably is over the top for simple saturation duties.

Reveal Sound's Spire?

http://www.reveal-sound.com/

Comparing to other tracks is the key you are looking for. Use a low pass filter on commercial songs and copare them with your song with a low pass filter.

And compare with a frequency analyzer, but what you hear is more important.

I don't get the logic. What difference does it make whether you're increasing or reducing volume?

If you have a muddy low end in a mix because the bassline and kick overlap in the same frequency spectrum, what do you do? Could out some frequencies or lower some frequencies to create room for each other, or add more volume to get more clutter?

I do the fader up thingy to get a rough idea of the mix.

Then I use the eq to get every sound clear, and then I do the fader up thingy again. I have the feeling indeed, that eqing can bring things out of balance a bit.

Thank you very much, cozmonavt!! Your comment makes my day!