What makes the biggest difference in sound quality?

System matching is the most important aspect of great sound. Having a system of equals works in harmony with everything else. Pairing a expensive pr of speakers with less than electronics results in frustrations and miss guided assumptions that some how the speakers are flawed. With this example your assuming the speakers are the fault because they sound less than perfect; when in fact their level of resolution exceeds what your other components have to offer. Balanced system is the key. 

I think quality speakers make the biggest difference. I know folks will go on and on about amplification, source, etc., but speakers have to be quality or the sound falls apart. 

I am interested in knowing more about how to check refrigerator compressor symptoms. How can I resolve this issue immediately? 

System matching is the most important aspect of great sound. Having a system of equals works in harmony with everything else. Pairing a expensive pr of speakers with less than electronics results in frustrations and miss guided assumptions that some how the speakers are flawed. With this example your assuming the speakers are the fault because they sound less than perfect; when in fact their level of resolution exceeds what your other components have to offer. Balanced system is the key. 

Good god, not yet another synergy and balance echo.

The single thing that made the most radical change to my system was installing (to audition from a dealer) a Mesa Baron (dual mono block in one chassis) in place of my Cary SLA 70 (which is a basic stereo amp using 4 6550s & a couple of 6SL7s and tube rectification).

The effect of the Baron in 1/3 triode and 2/3 ultralinear was intriguing; the music moved much closer and (for lack of an audiophile vocabulary) took on what I would describe as a musky, smoky quality. I listened to it pretty much nonstop for an entire weekend & was on the cusp of trading my Cary in for it. The last thing I did on the Sunday night of the audition was to put the little Cary back in the system. The sound stage got way smaller and further back, but what struck me was the truth of the cymbals shimmering against a black background on Cowboy Junkies (I believe "Sweet Jane") which I definitely did not get from The Baron. (I kept the Cary, which I still own.)

The next single biggest change (this one was definitely an improvement) was prior to the Mesa audition, I was using the Little Cary to drive a couple of NHT bookshelf speakers. I replaced them with a pair of B&W 805s (Matrix).

The third biggest change (& this was another definite improvement) was after the B&Ws and the Mesa Baron audition, and it was the replacement of a B&K AV digital preamp with a second hand Cary SLP 90 (which was finally upgraded about 7 or 8 months ago). At that time, it was in front of a couple of ARC VTM 120s.