The most important and most clearly audible system "tweak" is not really a tweak, but getting the A/C electrical polarity to all your components correct. When its backwards, a startaling loss of resolution occurs as well as soundstage losses. More aggressive component behavior is also a byproduct. A product called the NAMIKI direction finder, years ago, was available for this purpose, but I am not sure if there is a modern equivalent. Many power cords do not have a ground pin to establish automatic correct polarity. There have been cases though, where it has been wired wrong inside the unit.
Good tweaking tools to share
I share some tweaking tools with my friends because they are very expensive, seldom use them, but are essental to have periodic maintaince. or tweaking. Some of my examples are a Wind's stylist gauge, TT speed strobe, Fluxbuster, and a Mobie and others. Do you do this on certain items, what are those and their purpose?