Your room. The least sexy part of the process unless you can afford to hire a pro to do the whole thing for you. Most tweaks make micro differences, room treatment is a macro difference. Huge in most cases.
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.