What makes the biggest difference in sound quality?

I bet there is an astounding percentage of hermit Agoners. Wow. To the OP, Speakers and source components first. But reality is, that is a MASSIVE variable with all number of possibilities. No easy answer. Power has to be second. If power sucks, so does everything else. Speaker cables next, then on down the line. And I too disagree that the entire loom much match, plenty of people making great music with a variety of cables.

You can make DIY Cords and fuses as good, if not better than most manufacturers. Pennies on the dollars,

If true then you are making cables for pennies that are better than the ones I bought for thousands. If true you would quit your job as they would sell like hotcakes. I would be first in line to replace mine with yours. In fact you would be nuts not to send me a set, just so I would tell everyone how great they are and you really could quit your day job.

Which is what Lubos did, so we aren't just spinning fantasy here, it actually happened. 

Now I can hardly wait for the DM, to find out which ones of your marvels you want to send first. The suspense is palpable.

The room

Speaker position

Rack position

Dedicated line

Cable management

Everything else

Interconnects, speaker cables, power cords, or fuses?

I must not have good ears, because homemade interconnects seem to as good or better than most of the store bought ones I have tried.

Keeping speaker cables short helps, and also keep em fat in diameter and bi wire them if possible.

I have not ventured into powercords and fuses.

I must not have good ears, because homemade interconnects seem to as good or better than most of the store bought ones I have tried.

there could be an open forum for say, car guys. You could have a group of these car guys talking about how they reduced their best lap times by seconds while outlining exactly what they did to their vehicles to lower their best lap times. Another group of car guys come along to tell the first group they are all nuts and that none of their tuning tips work, some even claim to have tried their tips and that none of them lowered their lap times. The reality is, both of these camps could be correct. The problem is perspective and details. The drivers reducing their lap times could be Porsche and Ferrari pilots while the second group could be Miata drivers. Knowing the details of an audio system is extremely important to put anyone’s perspective in perspective.

As to cable length, shorter is almost always better to a point, but if you must have long cables, make them your interconnects and not your speaker cables.