What makes the biggest difference in sound quality?

It seems that this thread has brought out some important aspects about pursuing high end sound. Lots of things matter. The breath shows how everything matters… since individual experiences are so widely distributed. It also shows one of the reasons it is such an intriguing pursuit… you can continuously improve your system by working on the finer points. Speaker placement, room treatment, power cords, cable elevators… etc.

I completely upgraded my system about a two years ago. Just plunking down new components each created a “wow” moment. I enjoyed every minute as the component broke in. Then after some time, I would actively make a change… new interconnect or, power cord. One by one interconnects as the first confirmed my views. I was lazy. I didn’t get around to fiddling with speaker toe-in for over a year. My speakers were known to disappear… voila, they disappeared, soundstage increased greatly. Most other pursuits don’t offer such opportunity for improvement. While the character of my system has not changed a bit since I established the major components… the details of imaging, detail, soundstage… have improved dramatically.

Yeah everything matters, I still have to pick my jaw up off the floor after installing Duelund Bypass capacitors to my crossover. .01uf bypass for 10 uf, and 115.8 uf each. 

KRAY KRAY.....everything matters. 

"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?"

This possibly could have been phrased more clearly, but I think the OP meant that *of the options listed*, what makes the bigger difference. 

"And while power cords may make the biggest first impression, it is usually the last cable you replace, something like a mis-matched ethernet cable, an interconnect or a power cord going to something you don’t think matters, it is almost always the last cable that is perceived as making the single biggest difference when that cable completes the loom."

I think that this is the answer. In a highly tweaked system, it's the last thing you do that seems most significant, because all the previous improvements allow the most recent to perform at its best. Whichever order you upgrade your cables and fuses, the last one done seems most impressive. Or at least that's been my experience. 

As I learned today, I’ve been greatly underestimating the power of room treatments. I Installed a floor to ceiling bass trap in the most offending corner of my room and I’m astonished. Spent the rest of the afternoon recalibrating my sub to the “new room”. Best bang for the buck upgrade yet! 

As everyone has pretty stated everything matters and is important, but that is where i differ with most…

Let me go on the record as the outlier from the group with “everyone”.
Specifically with respect to the question of:
 

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? 

I believe none of the crap matters much, and certainly not the power cords and fuses.

But I may be a robot, and I believe that I am passing the Turing test.