Your most indispensable "tweak"?

Ear wax remover
Shunyata power cords
room treatments

Sorry for the plug... I have a PPT Mat for sale on Audiogon now.

ozzy

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.

A well implemented tone control in preamp stage.
Not to fix sound of system—rather, to tweak/render bad recordings listenable.