What makes the biggest difference in sound quality?

Especially if you have Lead in your ears. 

The question was framed in such a manner so as to specifically not ask about hearing, auditory perceptions or any possible psycho-acoustic effects.

This was possibly accidental, of course.

#1 - the recording quality 

#2 - my mood and how tired I am 

#3 - proper speaker placement in room

#4 - great amp & speaker synergy 

#5 - cabling 

@steakster Agree with  that everything matters, and speakers are the majority.

+1 @celestialsound 

1. Room

2. Speaker Position

3. Speakers

4. Source/Pre Amp/Power amp

5. Cables

Everything.