Your most indispensable "tweak"?

Trying to narrow my possibilities down... 

I don't know what constitutes a "tweak", but:

1- Room acoustical treatment.

2- Loudspeaker and subwoofer placement within the room.

3- LP cleaning.

+ 1bdp and to properly integrate the subwoofer to include the right crossover point and gain.
Power cables, interconnects and speaker cables.  Without them, I can’t get my system to work.
What did it for me more than anything else was a proper equipment stand (Sound Anchors) custom built to accommodate my anti-vibration platforms with my preamp on top of one, and the turntable on top of the other. The stand is in turn seated on a set of Aurios Pro bearings (unfortunately nla).

This combination allows the system to play at any volume and remain relaxed- even at 110dB (according to my Radio Shack sound pressure level meter).
@68pete what did these do for your system?

@needfreestuff I finally dialed in my sub to a crossover and gain level that opened things up so much. Not an easy task. 
There is a big thread on them here in the forum. They simple made the sound so much cleaner sounding across the board. Back around vocals where much stronger, able to see deep into the sound stage. More black space around the sound. 
Here is a few.
1. Sorcer X4
2. Critical Mass Footers under components
3. Iso Acoustics GAI 1 under speakers.
4. Stillpoints Aperture 2's

This used to be my FAVE tweak until one of my eyes fell out of grace (doesn't track that well following 9 surgeries).

Aside from that a decent equipment rack (even though my electronic gear is isolated by a double pocket wall from the listening room) as well as after market shelves for the source gear (Neuance/Greater Ranges shelves for the CD/TT decks) is way, way, way up there.

Cleaning electrical contacts for tubes and cables is also a big plus, but it's it's simply common/logical maintenance instead of being in the tweak catagory.


Dedicated 20 amp circuit  with an audioquest nrg power outlet and furman power conditioner. 
There is a big thread on them here in the forum. 
Can you link to this thread please?  Search does nor find it. 

Best of all:

2 dedicated audio lines with hospital grade outlets and 10 gauge wire. Was not expecting it to make a big difference...and it wasn't...but it wasn't a small difference either. It was a solid upgrade.

I had ordered a new tube amp when the lines were put in and I almost wanted to cancel.
I had a Carver with great high end but weak bass...and the bass was now sublime...overall sound was more focused.

At first, I thought it was the Niagara 1200 but the Isoacoustic Oreas turned out to be more than icing on the cake. I'll have to give them equal billing.

All the best,
Everything matters...I've been tweeting for years.  My last one gave me such an enormous change, it was like changing a major component.  I replaced regular plastic cover outlet plates to Carbon Fiber.  ..a huge difference.
I have a CD issued by high-end solid-state amplifier company Ayer Acoustics titled "Irrational, but Efficacious System Enhancement Disc". My dealer, now out of business(owner died), let me borrow it. Never returned it. It has white, pink, and 3 brown noise tracks. Brown noise is ideal for breaking in everything that needs breaking in: Cables, speakers, amps, DAC’s, etc. The last track is a 5 minute glide track that goes from 5Hz to 20kHz. Played at a moderate level, it makes your system sound better. It’s available at Ayre’s website. I’ve been using this disc for 20 years. I don't consider AC filtering components tweaks. Not anymore. Every reviewer in The Absolute Sound and Stereophile lists AC components in their gear. I have a long discontinued PS Audio Duet passive filtering device. Their AC Regenerators are bad ass, but Michael Fremer, a reviewer I respect, told me "I'm not plugged a power amp into a power amp." Audioquest has a good article on AC treatment. Of course the writer pushes his own design.
For me, the earliest and cheapest was adding a dedicated line (took one afternoon, less than $100). The most recent, and maybe best, was Krissy's boxes from former PPT: game changing. In between, early-1960 6085 and 7062 tubes (in place of 12ATx and AUxs) for the line stage of a LTA Zotl-10, which itself was a total ear opener. I've also been happily surprised with the SR HFT/FEQ treatment and IsoAcoustic Gaias. All in all, I've been amazed how much tweaks really do matter... I used to shake my head at my brother's Cryo treatments, but now I have an open mind: if I hear it, I go with it. I listen to single-driver speakers with Fostex F166e drivers (4.5") and it sounds like a concert hall. 
Well, after 30+ years in construction, and the hearing loss that results from that, I'm going to say "cupping my hands behind my ears". Not kidding, try it. Sound-stage instantly becomes all-encompassing, details are greatly expanded. First time I tried it, I was both amazed and a bit bummed. Take care of your hearing, should go without saying.
Dedicated Apollo turntable shelf, mass loaded Atacama speaker stands, QED biwire speaker cables, Audioquest NRG power outlet, Niagara conditioner, 20 amp dedicated circuit, AQ thunder power cable. Recently, Origin Live Gravity One record weight, yet to try, but looks interesting.
I run shotgun bi-wire cables with spades to my speakers, but adding jumpers in the banana connections as well significantly improved imaging and tightened bass to the point that I won’t listen without them. 
"Take care of your hearing"
Could not agree more. I built a porch on the back of our previous house during the summer of ’05. Circular and chop saw blaring every day that I was off from work. I ended up with tinnitus and still have it today. I think my hearing is okay as a couple of friends agree with what I hear on my system. But I sure wish I could go back to the summer of ’05 and protect my hearing.
So many interesting suggestions!  Thanks all.  And feel free to keep them coming...

@highend666 - thanks for the link, although completely impossible in my listening space

@stringreen - details of your carbon fiber outlet please?

@sjsfiveo - yes, couldn't agree more about NOS tubes

@soix - not sure what these jumpers look like in action -- can you describe more?

@barbapapa - not familiar with PPT -- what does that stand for?

@chrismini - super interesting suggestion

So many isolation/shelving ideas too -- will look into those.  Thanks!

Details of Carbon Fiber outlet cover.....this is the most idiotic thing I ever experimented with.   Surely the replacement of a plastic outlet cover would have no effect on the sound.   I even tried to load the inside of the cover with difference at all, so I left it in place.   The outlets themselves are audiophile approved Maestro, the cable is Neotech with top of the line Furotech carbon fiber plugs on each end.  On a lark, I went to Ebay and got 3 carbon fiber outlet covers for the 3 outlets in the stereo room.   The covers are from HongKong (although they came through California, they were 20 dollars each, and the change was very dramatic all on the up side. 
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.
My most expensive tweak was also the tweak that made the biggest positive impact. The $400 that I spent on two sets of Iso Acoustic Gaia III Isolators for my speakers was worth every penny.

"PPT" stands for Perfect Path Technologies. Their products are currently not available and I'm not aware of any recent news regarding if the company is coming back. Last year there were many threads on Audiogon regarding their products; so, there is plenty of info available here if you are not familiar with their products.   

Annual ear wax removal

I was going to say that too -- hahahahahahaha
My $49 Bosch laser measuring tool.  Another AGonger mentioned it (thanks).  Fastest way to get your speakers to "disappear". Check distances, seating position, aim for sure but toe in and tilt too.  An inch matters.  Really.  

Cabling you can afford but not crazy.  

Been said before, ear protection for all loud activities. This is a life time hobby.