Power amplifier listening tests must be the ultimate arbiter of sonic quality, but getting there depends on a lot more than random tweaks. Here we discuss techniques for measuring distortion and provide discussions that help understanding of distortion. Distortion measurements are certainly not the be-all and end-all of amplifier quality, but they are nevertheless very important. I believe that when proper techniques are employed to reduce measured distortion, those same linearizing measures taken tend to increase the overall sonic quality of the amplifier, even if those measured distortions that were reduced are of questionable audibility. Think of distortions revealed by measurement as symptoms of amplifier dysfunction, rather than the audible bad result itself.