1. Attitude. I've been moved to tears listening to music on a cheap transistor radio.
2. Speakers. Assuming you have a reasonable set up (whether inexpensive or otherwise), better speakers will make the most obvious improvements.
3. Room. I'll venture to say most people have little control here, and are unwilling to move their speakers or spend enormous sums on room corrections. For those who are willing, a good room can make a mediocre system sound good and a good system sound great.
And the biggest influence on sound quality is...
The quality of the recording itself.
Then the room, the setup, the speakers, and lastly the front end.
I've got recordings that make my system sound horrible, and I've got recordings that make my system sound absolutely wonderful.
None of the gear changes have had that much impact on sound quality.