What makes the biggest difference in sound quality?

To answer your specific question, in my system, of the items you listed, I believe power cords have the biggest impact on the sound quality of my amplification components.

Surely, new speakers and new cartridge made the hugest difference.

However, the most shocking and, by far, best bang for the buck was Townshend Podiums under my speakers. The character of the sound radically changed for the better. Shocking.

@steakster 😂😅