All of my upgrades have improved my audio system. The best upgrades, in my experience, in order of audio improvement were:
1. Speakers (3-way Floorstanders to Open Baffle plus subwoofer)
2. DAC (++ quality & expense)
3.Preamp (SS to tubes)
4.Amplifier (SS to tubes)
5.Speaker cables and Interconnects (tie)
6. Room treatments (but I already had a good room, as-built)
7.Power regenerator (but I already had good power)
8. Power cord upgrades
Your experience may vary. Everything matters, but not equally!