I recently added the QSA Red and definitely heard an improvement in the sound quality. I'm using it on the power cord for my Gigawatt Powerprime power conditioner. I noticed better separation between instruments, a bit more presence/oomph, and definitely an appreciable difference in bass -- tighter and punchier. In fact, the improvement in bass response is such that I have switched off my two REL subs and can still enjoy jazz, vocals, classical. I only turn on the sub when listening to rock, EDM, etc.
Having said that, I also have a QSA Red Receptacle so not sure how much of a role is it playing here. I don't have any experience with the Yellow or Violet.