I used a green pen back in the day, the "change" it made wasn't that obvious so I stopped using it. I've tried all sorts of tweaks, custom cables, replace all the wiring in an amp with silver cable, replacing coupling caps with better caps, rolled tubes, different isolation techniques, De-magnatizing CDs, changed cables, build my own cables, converted pentodes to triodes, installed a custom star ground and separate circuit in my house house for the stereo, all sorts of things- I heard differences is almost everything I tried; however, I came to realize that my perception of differences in audio weren't limited to times when I changed something in my system.
Have a couple of beer and things sound different, listen late at night things and sound different, too much coffee, stressed out and guess what? Sounds different. It just didn't seem reasonable to believe that my perception was a valid way of determining the truth of system changes. Tweaking and optimizing was pointless, I was chasing my own tail. I became a cynic not from measurements and graphs but from following my own experience of listening.