How much better? So much so that if they were behind a curtain and you couldn't see what they were stood on, you would be confident that you could pick out the stillpoint every time?
When I was still doing tweaks like that I always used that as my benchmark. I think it sounds better with the tweak, but would I be confident about spotting the improvement blind? If the answer was no, and there was no established explanation for there being an improvement, then I'd dismiss what I was hearing as a psychological effect.