What makes the biggest difference in sound quality?

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.
 

I also didn’t ask what people had for Christmas dinner.

@ebm 

Everything

+1

For me, the updates that have had the biggest impacts to my preamp and amps were power cable for the preamp and speaker cable for the amps. The power cables  for my amps also made a difference, but not at the same level as the speaker cables. I recently added spring isolation to my amps and that added more depth. I think it helps since I use tube amps.  

I've found speakers > amps > source > isolation >  cables.. Within all this the poorest performing link will place limitations on the best. For example, a great interconnect will get murdered going through poor speaker cables. Trying to make sure it all matters is important, or at least keeping that in mind as you go as your building a system. Ted, can you explain some of this from your own experience?  

Interconnects- significant positive changes.

Speaker cables- less significant positive changes.

Fuses- no changes. 

Power cables- less significant negative changes.