I have used the VST saturation plugin called Saturn and discovered how easy it is to use certain effects on some frequencies and other effects on other frequencies.
I would like to add effects on just some frequencies and allow the rest of the frequencies to be out of effect.
I do it all the time on vocals and bass.
Simplest way, duplicate the track and use filters to to only allow for the mids to be heard.
Then add a healthy amount of saturation and fade it in til you hear the vocals fattening up.
And then there is quadrafuzz, that is made for that kind of stuff.
I often use the build in delay on a single mid band, combined with the ducking feature of the delay it can be really subtle if need be.
