The cable should sound open, airy, and extended in the treble without sounding overly bright, etched, or analytical. Similarly, massed voices can sound glazed and hard rather than liquid and richly textured. The symptoms of listening fatigue are headache, a feeling of relief when the music is turned down or stopped, the need to do something other than listen to music, and the feeling that your ears are tightening up. This last condition is absolutely the worst thing any audio component can do. If a cable or interconnect causes listening fatigue, avoid it no matter what its other attributes.
Poor cables can also make the soundstage less transparent.
Some cables and interconnects sound smooth, but they obscure the music's fine detail. Listen for low-level information and an instrument's inner detail. Musical detail should be audible, but not hyped or exaggerated. The cable or interconnect should strike a balance between resolution of information and a sense of ease and smoothness. With such a cable, the bottom end is soggy and fat rather than taut and articulate.