I use Oyaide C-004 on IEC and Viborg Silver/Rhodium on Schuko in my system. My observations is that the plugs not only have an effect on the frequency repons (less warm, more neutral) but also on the time domain. The hard metals don't have the "soft" 'decay' of gold and silver and present a tighter sound with more 'attack', punch and control. They suit my taste of music (rock/pop), but I understand that not everyone likes them.