No instrument or scope can measure whether a power cord or interconnect sounds good. For that matter, neither can these measures exactly how good a pair of headphones may sound in the real world. The gold standard is the human ears. It is ultimately how the ears perceive the sound. We are not robots or androids. There are basically 2 approaches to designing a hifi product. First the measurement method of frequency response to achieve a theoretical ideal best. The second is the golden ear approach in which live humans craft the sound which in their best opinion and knowledge constitutes the best sound. So, interconnects and power cables are mainly made using the golden ear approach and never by just measurements. Similarly, the recording process is affected by the recording engineer who uses his many tools to craft the sound which constitute the best sound in his personal professional opinion. It is never flat frequency response per sec he is after. Even if we are purists and don't use any equalisations or tone controls, the music played is never flat as the source is already somewhat manipulated by the recording engineer. Interconnects and power cables are sometimes perceived as having "a tone control" effect on the sound. Old timers will tell you that different cables of different manufacturers have different sound as if different tone controls are used.