Upgrade with best price/performance ratio

Just wondering what was your upgrade with the best price/performance ration in your audio journey ... like upgrading an IC, preamp etc ...

Let me start:

-Corian pieces leftover from kitchen upgrade project

out of boredom I used the Corian chopping boards and put them in between the shelf's MDF board and BDR cones. The effect was a big surprise ...

- much quieter background, solid bass and cleaned up mid and treble.