Most eye-opening upgrade and what you learned

While I've made a number of upgrades and tweaks along the way to building a good system, the two upgrades that come to mind as truly "ear-opening" are (1) purchasing a pair of audiophile grade speakers (PSB Stratus Gold-i) to upgrade a quality mass-fi system. The huge increase in musical enjoyment bowled me over and awakened me to the possibilities of high resolution audio, getting me hooked in the hobby. (2) adding a Tact RCS 2.0 room correction processor to the system. The surprise here was not so much the delightful and substantial increase in focus, resolution, and smoothness of the sonic experience, but the realization that so much of what I thought was whomping bass capability in the system was actually muddy room reverb... bass drum hits changed from a big, meaty "boom" to a tight thump with microdynamics of skin reverberations. Bass guitar lines became distinct musical notes. The rhythmic underpinnings become so much tighter, so much more "there". Listening to clean bass takes getting used to... at first the sound seems thin, until you bypass room correction processing to hear the mud again.