Most eye-opening upgrade and what you learned

The REL line of subs, a staggering revelation about the potential and the role of subwoofers in high-end audio. If you had asked me a decade ago whether I thought subs "belong" in a high-end system, I would have laughed. Not anymore. They have literally taken me all the way from "no way, not ever" to "indispensable in recreating the original musical event". You owe it to yourself to audition a properly set-up system using these subs (preferably at the Stadium level or higher, and with a high-quality two-way speaker). And make sure the person who tuned it knew what they were doing, or else you're wasting your time. When set up properly, they will simply vanish, and it will sound like the partnering speakers extend down into the center of the earth, and the listening room walls evaporate into thin air.

Mine was when I installed a Tara Labs power cord on my system. This was a smaller aha!!!, but shocking nonetheless.

Shouldn't it say: EAR opener? :-)
Revel was the same for me! Ended up in purchasing the ULTIMA SYSTEM, w.i. with the Sub15 - better than the Salon.
Involving tubes was another big step for mankind (McIntosh
MC 2000)
Among the "cheaper" steps were the PS Audio Power Plant (psaudio.com) and Kimber Select Interconnects and speaker cables.
The latter again create MUSIC instead of sound - as you described it perfectly.
Good luck!