I know at least the older Aural Exciters, would take out some of the higher freq. and add distortion then mix it back in with the main sound, infact, I forget who it was but one person had to stop selling kit's that did this, because of copywrite, but he did have a article in a mag showing how to make one. I had a BBE back in the early-mid 90's, back then they said they split-up the signal at certain freq. and re-aligned them in the time domain, then put them back togather. Are they worth it, as an effect they can be, but they can easily get over used.