Several over the last couple of years ...
- Buying a laser measuring device and using it to properly set up my speakers in the room. Now, I have Apogees, which need as precise a set-up as possible, to within 1/4-1/2 inch. This allowed me to accomplish that.
- Tacking a quartz crystal to the front of my phono cartridge - aka Black Diamond tweak from Walker audio. You simply have to try this to believe it.
- And this one we all know, but not all of us actually do it ... clean all your terminals and connections once a year. And while you are there make sure your cables are dressed as properly as possible.
- Room treatments - proper absorption and proper dispersion. Enough said.
- Turning my down-firing REL sub upside down so it is now up-firing (with a board placed on the up-facing feet) and re-balancing it's output. Much quicker, faster, better integration with the fast ribbon speakers.