What makes the biggest difference in sound quality?

@ted_denney Ted, since you framed the question to cover all the products Synergistic Research sells, I can tell you that the Shunyata cables and power distribution devices I recently purchased made a significant SQ improvement over your cables and PowerCell device, especially for my dCS Rossini DAC/Streamer. 

It is very disappointing how many people didn’t read OP’s question and answer in a constructive fashion which of those 4 items made the biggest difference for amps and preamps. 

I think it may be a potential adverse reaction to self promotion.

@boothroyd  +10

To my ears, I would have to say that going from stock power cords to a nice one like the Audience Power Chord, or one of Morrow's line on the power amp made an audible difference in the tightness and presence of the bass.  Speaker cables made a difference as well, audible but subtle, more in the high end of the treble. For me, IC's made the smallest difference.  But YMMV, since I don't have "golden ears" anymore, unlike some of you folks . . .