And the biggest influence on sound quality is...

The room+speakers, then everything else. You’d be surprised how good some “bad” recordings can sound with high-end speakers in a proper room.

1. Gummie

2. Ears

3. Wallet

4. Good wife

5. Room

6. Recording 

7. Speakers

8. Preamp

9. Time....maybe that is number 1.

The last thing I changed.  (if I got it right)

Seriously, though, I'd have to say speakers.  Every time I change speakers the system gets VERY noticeably different.  I've made small, to the extent I can, modifications in the room and there is impact.  But admittedly I'm limited.  Bride has nixed taking out the wall and the stairs on the other side of it.  Also, won't let me blow out the bedroom upstairs so we can have a higher ceiling in the listening area.  MAN!

Amps have peeled back layers, brought more speed and clarity and with them nuance.  So have wires/cables.  The Holo DAC was both subtle and powerful.  The Townsend speaker bases were the biggest surprise.  They very noticeably impacted the treble, ion a good way!  Still don't understand that.  Are speakers the most important thing?  I don't know.  But I'll say, in most normal situations, the speakers have the biggest impact.

These kinds of questions are like asking what it the most important part of a bridge.

None of the gear changes have had that much impact on sound quality.

Seriously?  What gear changes have you experimented with so far?  I have heard profound changes with different pieces of gear.