What makes the biggest difference in sound quality?

+1 speaker positioning and the room!

@borat-sagdiyev 

None of those things. Better amps and preamps are where you go next, or room treatment, Dirac live, etc.

It is a bit of leading question, as the OP has a company that sells the fuses and the power cords.

But I agree with the EQ part as clearly having an objective impact.

90%++ of the time, the source will improve the overall sound of an ok system, before anything else

You are right for the fact, wrong with the 90%........

Try the same source in two different room and you will know the importance of acoustic over almost anything else...

Like in a car tires are important but the motor import more...

What we hear come from the speakers/room for most of the S.Q. %...

Acoustic is the most important factor , engineereing is the SECOND factor...It is the opposite that what people think because we must buy an audio system first and chose one before treating and controlling the room for a specific system...Then we think that picking the system is all there is...

Why is acoustic the most important?

Because ANY relatively good design will play with a difference between them which will be way less than the difference between acoustically controlled room and a bad room ( almost all untreated and uncontrolled rooms)..

If i am wrong, then why am i satisfied with a 500 bucks system? It is not the best there is at all then am i deaf?

No it is not the best audio systewm at all, only a good one, but my room is so well controlled than a relatively good system at low price (vintage) sound extraordinarily good... So good upgrading is not appealing because of the ratio S.Q. /price scale diminishing return...

If i am deaf why my 7 headphones are in a drawer definitively?

Because acoustic is the sleeping princess and all pieces of gear are the 7 dwarves...

90%++ of the time, the source will improve the overall sound of an ok system, before anything else

It’s been said that everything matters.

When I walked into the home my wife wanted, I walked into each room and clapped my hands together. The lounge room was about as inviting acoustically as the bathroom, just terrible.

I put what I had in the best sounding room, even had my wife’s blessing.
So I’ve been here on Agon, and read from many of you on where to go and perhaps change my assumptions and thinking. It’s certainly paid off, really has made an improvement over the last couple of years.

Filtering the quality of my AC power, cleaning up the sinusoidal waveform, was quite a revelation. I had already known about isolation of speakers, I invested in some experiments of my own, discovering a spring technology that I had not been aware of, along with spikes for my stands - marvelous.

The latest and not surprising was my admittedly novice journey into room treatment. Indeed just a couple of quadratic diffusers made quite a substantial difference.

Some of us (two thumbs pointed right at myself) have a limited budget, and as such, heed to advice from others in here. Now that some fundamentals are in order, the room itself appears to be my next best investment for a more beautiful presentation of my music.

I would guess, that identifying good value for money items that play well together, and finding the right synergy to get basics sorted out, like clean power, relatively good in room frequency response (loudspeakers must reproduce the entire audible spectrum of sound faithfully) and taming the room can trump the sound of a system where something is simply lacking in the basics (irrespective of cost).

90%++ of the time, knowledge not money will yield superior results, knowing where and on what to invest that money/time, now that’s the real art. Whether you are paying for someone else’s knowledge or not, I think my statement has merit.