What makes the biggest difference in sound quality?

Typically there is a hierarchy of speakers, preamp, source, amp, cables, interconnects, power cords… but in any system they can get scrambled up depending on all sorts of things (components, room acoustics). But while there may be a hierarchy, they absolutely all matter. So I typically attack an upgrade in the order I show above.

speakers always most significant,, 

thats the majority vote

in my tube amps, I'd say the tube more than design/quality of trans

2A3 vs KT types

THen KT88 vs KT90

Then one brand 88 vs another (I did not care for Gold Lion) 

2A3 solid plates vs 2A3 perforated (aka mesh) plates. 

Seeing how many speakers are for sale, constantly never ending, I believe the speaker / room / listener situation is number one, and must be diligently addressed. And, if you do not have a dedicated music room ( I apologize to all that do not ), and had a living room for audio and social gatherings......lots of luck ( I am sorry, but this has been my experiences ). Merry Christmas to all, and be well.

Interconnects, speaker cables, power cords, or fuses?

for me, ic’s (i believe that is what the op asked)

i haven’t tried fuses (nor am i planning to)