Power cord replacement where first?

Do a search on shielded versus non-shielded then experiment.

Looking forward to your impressions regarding which component you think has the best result with the PC.

The digital source is what I'd suggest, the upgraded power cord will both improve the input to the CD player and also filters the digital nasties that are outputted into your power supply from the CD player. Two for one. None of the other components need both input and output improvements from the PC.

Like Eliz says, try it on both. Let us know your thoughts.