Best Upgrade you had?

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).