Group delay is phase distortion (see also Wikipedia). Best is none. So the silver cable in the graph is less accurate in this area and in this respect than the copper cable. I'm puzzled that there can be such a strong measurable effect in cables. Have you also measured other (copper, silver...) cables? Group delay at low frequencies isn't necessarily a hint to accentuated bass or attenuated treble, it's just phase distortion.
2/17/15 at 5:20am