The more I listen, the more confounded I am by perceived changes in the sound quality of my system when I have made no changes. And it's not just related to my mood, atmospheric conditions, what I'm wearing, or (as far as I know), my physical health. My power supply is robust and consistent. I played in a range of musical groups for many years, I attend live concerts - both acoustic and amplified - and I'm an experienced hifi listener.
I quickly recognized the difference that switching from my NAD M32 amp to my Modwright KWH 225i made, and that improvement has continued to be consistent, but there are days when the same piece of music sounds open, three-dimensional, and engaging, and other days when it's just "there."
Because of this, I don't trust my ability to evaluate "tweaks" except over the long term.