What makes the biggest difference in sound quality?

Your thread isn’t working out quite as hoped aye Ted 😉

Interconnects, speaker cables, power cords, or fuses?

None of the above.

None of the above

Especially if you have Lead in your ears. 

1. Room

2. Speaker Position

3. Speakers

4. Source/Pre Amp/Power amp

5. Cables

Especially if you have Lead in your ears. 

The question was framed in such a manner so as to specifically not ask about hearing, auditory perceptions or any possible psycho-acoustic effects.

This was possibly accidental, of course.