Your most indispensable "tweak"?


Trying to narrow my possibilities down... 
redwoodaudio
Definitely dedicated 20 amp circuit. I don’t think this is a tweak but adding a minidsp to eq my subs was also a game changer
I don't know if everyone will consider this a tweek, although double shotgun speaker wires were mentioned, but for me the best tweek has been switching to the "Doug Schroeder Double IC Method".  Nothing has come close in SQ to doubling up my interconnects.
I wired in a double pole double throw switch to sometimes bypass my sub crossover.  This allows a lot of harmonics to be added to old LP recordings that are otherwise terrible sounding.