Best Upgrade you had?


What single upgrade did you have that gave you the most impact in sound improvement? Mine would be the EMM LABS DCC2 dac and CDSD transport followed perhaps by my AUDIOMAGIC line conditioner.
jjwa
Being bumped to first-class on a transatlantic flight out of London. It was dramatically quieter, and the sound emanating from the headphones was far superior to that suffered by economy class passengers using those throw-away things. I would say, that in comparison, the sound was airy, incredibly life-like and almost palpable. I felt as if I was in the clouds!

At home, the single upgrade that impressed me the most--and first opened my mind to the possibilities of better sound--was buying a pair of heavy Target speaker stands for my LS3/5As (which previously sat on a bookcase). Soon after I discovered the differences cables made. Then I replaced my Denon direct-drive with a Systemdek. Then room treatments, etc.

The speaker stands, however, remain significant. As a musician (musicians often have dreadful stereo systems) I had been highly skeptical that such changes would make a difference. And like many others, I was entirely convinced (more like opinionated, narrow-minded) that was I was hearing was all there was to hear.
In 1983 I sold my very "very high quality" SS preamp and bought a ARC Sp10II. No single component has come even close to yielding that great an improvement in my home. Next to that improvement, the next greatest improvement came in fine tuning speaker placement (down to the inch and degree) and listening position along with appropriate room treatment to control reflections.
This is 10 pages long, "just kidding." Maximum Super Tweeter
Townshend Audio extend the responce of your speaker system to 100khz. If you got a box speaker, it's a no brainer. Put it in your system for 2/3 hours of listening, and now take them out. It's truly amazing , these have a 10 point settings, I think that is a must! $$$$ Note: to many others to list.
Shunyata Hydra 8 with the Python power cord. It makes everything plugged into it come alive.
Dedicated power brought my system to a new level (cost-$600). 2nd-my Bat vk5i with nos tubes - eight 1963 and 1964 matched 7308 Amperex PQ white label w/ shield small signal tubes and two 1957 5881 Tungsol power tubes (tube cost- I try not to think about it).