Saturation is a very powerful tool at the mixing stage, and in this article I’ll cover the most common uses.
How-to
Adding punch to the low end
1-2dB of saturation can help the kick and the bass to cut through the mix. Keep in mind that saturation acts as a compressor and will eat up the transients. Use parallel processing to get the best results.
Enhancing the vocals
Depending on the timbre of the voice, you can either saturate the entire track or select specific frequencies to create higher harmonics.
Mixing in the returns
Add some saturation to your spacial effects bus to help it blend into the mix.
Parallel processing
Try applying saturation via the parallel chain for more control, e.g. to process a selected frequency range or to preserve the original transients.
Which plugin to use?
There are many plug-ins that emulate specific devices (e.g. tape saturation) or do generic saturation, but it all comes down to soft/hard clipping, the range of harmonics produced, EQ/filter and speed of response (attack, release). I suggest you start with a plug-in you already have in your DAW, such as Overdrive in Logic Pro, and take it from there.
This technique is used:
Genres: all
Instruments: all kick bass vocals
Stages of production: mixing