What is the best tweak?

Alternative: skip all $$$ tweaks: No $1,000. cables, no $700 powercords, No $300 jars of rocks... no $200 cones...
Save all that money, and just get better primary equipment in the first place.
With better equipment, you can skip the super tweaks... in fact you can use cheaper interconnects and power cords, and STILL HAVE BETTER SOUND, than with lesser equipment, and all the tweaks in the world.
Yes I am saying, for the money spent, a better grade of basic gear is worth WAY more, than a lesser grade of gear with money thrown at a pile of tweaks.
My (humble) $0.02/experience from 38 years of audio insanity

Geometry..No such thing as isolation ..Dampening sucks the life out of the music..Geometry redirects the musical performance to the listener and can redirect resonant energy to ground. Preserve the performance..Long live the music..Tom

Tbg...how to spel it. ;)

Ditto to Elizabeth's idea......Many paths to salvation fortunately.....Peace,Bob

First best tweak is washing all your vinyl with a good quality cleaning system. Many excellent systems exist. For the budget minded, the Disc Doctor system with manually operated vacuum machine is outstanding. You won't beleive your ears!
Second is running your entire system on its own dedicated electrical line from the breaker box. From a 20 amp breaker run 10/2 Romex up to two high quality 'hospital grade' outlets. Your components will appreciate their own food source and reward you with improved sound!
Regards,