Your most indispensable "tweak"?

Symposium Acoustics Rollerblock Jr+ under my turntable and this all on a Symposium Acoustics Segue ISO platform.  Stunning improvement on bass notes, all percussion, and a wider/deeper soundstage overall.  So much so that I have since added more Rollerblock Jr+ for my other components and now have on order a Symposium Acoustics Foundation Rack Ultra.  I also found very good improvements by adding in a Niagara 1200 power conditioner and NRG wall outlet and am using AQ Tornado power cables.  Then added a dedicated 20 amp AC line.  Then upgraded to AQ Earth signal cables.  All of these upgrades (tweaks, as you may wish to call them) made very good improvements each and every time.  So, had to do another.  Recently went up the ladder in speaker cables with AQ William Tell ZERO.  WOW!  That is one heck of a cable.  Upgraditus is suggesting to me that I am not done.  Good fun, no doubt.

Tweak(s) of the year:

1. A tie:  ADD-Powr ElectraClear Harmonic resonators
              ADD-Powr eau2 Harmonic resonators

2. ADD-Powr Sorcer X4 AC Harmonic resonator

3. ADD-Powr Wizard AC Harmonic resonator

A no brainer.

For less than $500, the Akiko Audio Harmonizer Unit is the best tweak that I have ever experienced. It transformed my system above and beyond what I hoped it could do. It is also the most straightforward tweak to install because it's connected to the IEC ground in my power conditioner. 

Vocals sounded 3-dimensional before adding the Harmonizer Unit. After adding the Harmonizer, the music sounded REAL! It gave me goosebumps. The noise floor vanished, and the soundstage layering is now phenomenal.

OP - jumpers are just a pair of mini cables about 3” long with banana speaker connectors on the ends that bridge the left and right upper and lower binding posts.  IME they don’t need to be the same as the speaker cable you’re using — they just need to be decent quality, and at these lengths it’s a relatively cheap tweak.  I’d guess AudioQuest and others sell them at those lengths that you can probably buy from the likes of Crutchfield, Audio Advisor, Music Direct, etc. with a return policy so you can try them at home.  Mine were made by Chris Sommovigo of Black Cat, and he was the one who recommended I give this a try and very glad I did.  FWIW...

Much of what is mentioned above, imo, are not tweaks, but actual components and necessities, as part of our systems, for bringing out the best for our enjoyment. I am in line with atmasphere ( even though we are on different sides about some things ), that our systems can easily deliver peaks at 110 db ( neither of us are deaf because of this ), and at these volume levels, acoustic airborne and mechanical vibration, does need to be dealt with. To add to this, if the gear is affected by these volumes ( share the same room or location ), they are affected, not just from below ( stands, shelves, supports ), but from all around the components, and there are devices and tweaks, for on, and inside, the components themselves, that can be purchased / used, that can, and do,  help. The glass of the tubes have got to be dancing a bit, even with tube dampers installed, ime.