What makes the biggest difference in sound quality?

When making changes or adding things to your system, what makes the bigger difference in sound quality on preamp‘s and power amps? Interconnects, speaker cables, power cords, or fuses?

Key phrase is 'making changes': save your money and just toe in or out your speakers a few degrees to achieve your desired effect.

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.