Most useful tweaks that are sensible and really make a noticeable improvement

 Check the bolts holding the individual drivers in your speakers at least once a year. You might be surprised how they can loosen over time...

Best tweaks are to have best components including cables. 

Gong!

clearthink muddies:

If you are going to do this for the purpose, goal, and objective of higher fidelity then you will also have to design, build, and install a clamping device to insure that your head never deviates from the optimum measured listening position.

Actually if you think about it clearly you will see that even though we measure to a point that is for convenience. The sweet spot is in fact only one point along the imaginary plane that runs equidistant to both speakers. Move anywhere forward or back, up or down along this plane and you will still get good imaging. I even caught a guy doing this one time in my room. He got up out of the chair and was slowly moving closer and closer to the front, stopping every so often to move his head a little to one side or the other. Eventually his head was right above the amp, which sits on the floor between the speakers. Actually a little further back than the speakers.

Its interesting this guy who never heard soundstaging before the minute he did was able to figure out what some who devote tons of time and money and consider themselves audiophiles still do not seem to understand.