I'm going to be controversial and say that I heard a clear difference in the Sennheiser 580 stock cable and a replacement cable I got.

That said, I think you would be far better served by getting a headphone amp. The 650s are very power hungry (your average headphone has a 32ohm-64ohm impedance - the 650s have a 300ohm impedance), and your regular old headphone jack might not provide enough juice to adequately power them.