Best Upgrade you had?

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).