What makes the biggest difference in sound quality?

Program source. Junk in, junk out.

@STEAKster! I was going to respond to your comments, then I saw your OFFENSIVE and 100% IGNORANT username! I am 36 years VEGETARIAN, now VEGAN. I am also a HERMIT because I REFUSE to support our SATANIC SLAUGHTERhouse society. I also KNOW that many of you on this type of forum trend towards NARCISSISM, and DO NOT CARE. I'm also an ex-hockey player who still trains regularly, NOT one of those WIMPY liberal, commie girly-men vegans. I could have chosen to IGNORE this, and kept QUIET, but my CONSCIENCE will not allow that. 

I'm with AC and room as foundation. Of choice Ted gave, power cable>speaker cable>digital/IC> fuse

The first five things are the room.  Everything else you listed comes after that.

Let me clarify a little.  There are people who train themselves to hear past the room and only hear the equipment.  That's an amazing feat of ear/brain effort.  For everyone else, the room is the biggest thing, then speakers, then amp, then whatever.