And this, from my ARC REF 210 Owner's Manual:
IMPORTANT: Use the best available speaker wires and
interconnects. Audio Research cannot emphasize this
enough. As better components and systems are developed,
it becomes increasingly important to avoid the limitations
of inferior system interconnections.
If Audio Research cannot emphasize this enough, I will listen to their advice, and I do. I have always heard significantly better sound when placing higher purity copper, plugs, etc.