Most useful tweaks that are sensible and really make a noticeable improvement

So after reading the thread of useless tweaks I'd  be interested the communities opinions of useful tweaks. I may be rehashing a previous thread but times change as do useful tweaks.

I have found that in my case the following were useful, immediate and audible,
In order of priority in my opinion
1 Room treatment
2 Speaker location, ie proper setup
3 Subwoofer location (if used)
4 Subwoofer integration
5 Component isolation
6 Cables, all SC/IC etc, normally I would not mention cables but did have an ear opening epiphany that makes me believe cables are system dependant and I do not mean directionality.

So if anybody has anything to add, please do so as none of us are ever too old to learn.

Depends on whether a turntable and phono amp are in the system. If they are, then the platform under the table is critical. Rolling tubes can also be transformative. Always remove the lid from a turntable when playing and removing the cover from preamps can also open the sound. 
Not sure if you consider Cone feet part of isolation, but from what I have read they drain vibrations from component cases as well as isolate.

Also, if you are into vinyl and want to get the best from a cartridge that has a plastic case then try this tweak

Make sure that adding the brass plate does not exceed the compliance of your arm before you try this
Everything sounds better cryo’d. Even home freezing is highly effective and permanent. Two days in the freezer and 6 hours in the refrigerator section to thaw slowly. Theoretically 6 hours in the refrigerator prior to the freezer might not be such a bad idea, either. You know, thermodynamic symmetry and everything. Personally I skip both steps in the fridge.  😬 CDs, LPs, cables, CD players, preamps, tonearms, cartridges, power cords, integrated amps, what have you. This is very hush hush but there are others things to freeze at home or to send off to Cryo Lab but they’re beyond the scope of this discussion, they’re more for the advanced tweaker, so to speak.
Five steps:

1. Room. Get it under control.
2. Control micro vibrations.
3. Control micro arcing.
4. Purify the house power.
5. Sit back an enjoy your new audio system.

Here are a few actual tweaks that are dirt cheap...
Cleaning the contacts of AC prongs, RCA ends On copper I first use stuff I have laying around, Meguiar’s Scratch X. Does a great job removing tarnish, and polishes the metal. Then clean off with alcohol. Good to use a contact enhancer like Caig Gold to protect the freshly cleaned surface. a bottle of Caig Gold liquid (100% solution) cost $35 and last.. (remember I use it all the time) lasts TEN YEARS. (if you do not spill it).
Another free tweak, Use cut in half, with a notch empty toilet paper rolls, or paper towel rolls as cable risers. did I mention free. I also have used wood blocks even baby alphabet blocks do well, glass jars, anything that can lift the cable off the floor.    
One way to separate piles of AC cords is to wrap them with pure cotton rope. It is a lot of work to spin the rope around a zillion times. For the ends I use a zip tie and just zip the end to the next bit. "good enough" and allows me to tighten up the wrap if I want to by just twisting it a bit more. Gives every AC cord a little extra space. I use half inch rope. This is a a but expensive, but the wrap lasts forever, is reusable...
Some of us are well beyond this now, but it never hurts to repeat the most cost-effective tweak you can buy is a tape measure to perfectly align your speakers exactly equidistant. My beautiful example of this is the time we were struggling to get good imaging in the Talon Audio room at CES. After more than an hour of tweaking I finally said let's forget the room and walls and just measure the damn things. Ten minutes later and even I was shocked how much better it sounded. Yeah! Science!
millercarbon"it never hurts to repeat the most cost-effective tweak you can buy is a tape measure to perfectly align your speakers exactly equidistant."

If you are going to do this for the purpose, goal, and objective of higher fidelity then you will also have to design, build, and install a clamping device to insure that your head never deviates from the optimum measured listening position. Do not tighten the clamp too tight or injury may result, which may explain some of the postings common recently on this site.
The only scientifically correct way to find the absolute best locations for speakers is using
the out-of-phase (speaker set up) track on XLO Test CD or similar Test CD or Test LP. All (rpt all) other methods, e.g., making speakers equidistant, making an equilateral triangle, using lasers, trial and error by listening, will be unable to find the absolute best locations. It would be like looking for a needle in a haystack. When the system is out of phase you should hear the sound coming at you from all around, with no particular direction. When that occurs, and it won’t happen overnight because the room acoustics if not treated interfere with how perfectly diffuse the sound can be on the out-of-phase track. Rome wasn’t built in a day. Anyway, be that as it may, when you get the out of phase track correct you will get the best sound when the system is IN PHASE. Everything else is a trade off. In some cases because the room acoustics are so bad, you might not even hear ANY differences when you move the speakers around. No one said it was going to be easy. Other locations might sound OK, but they won’t sound as good as the locations found using this out of phase method.
If you are going to do this for the purpose, goal, and objective of higher fidelity then you will also have to design, build, and install a clamping device to insure that your head never deviates from the optimum measured listening position.

I think what @millercarbon meant was making sure the speakers are equidistant from the back- and sidewalls. That will have a big impact on room acoustics regardless of the exact listener position.

Yes on the tape measure. I found a good position and wrote down the exact distances from sidewall. right edge to rear wall. left edge to rear wall. I cannot tell you how many times I have used the measurements to put back my speakers after having to move one or both.
+1 Exact speaker position measurements (Qual ESL57...cough...)
 Check the bolts holding the individual drivers in your speakers at least once a year. You might be surprised how they can loosen over time...
Best tweaks are to have best components including cables. 
clearthink muddies:
If you are going to do this for the purpose, goal, and objective of higher fidelity then you will also have to design, build, and install a clamping device to insure that your head never deviates from the optimum measured listening position.

Actually if you think about it clearly you will see that even though we measure to a point that is for convenience. The sweet spot is in fact only one point along the imaginary plane that runs equidistant to both speakers. Move anywhere forward or back, up or down along this plane and you will still get good imaging. I even caught a guy doing this one time in my room. He got up out of the chair and was slowly moving closer and closer to the front, stopping every so often to move his head a little to one side or the other. Eventually his head was right above the amp, which sits on the floor between the speakers. Actually a little further back than the speakers.

Its interesting this guy who never heard soundstaging before the minute he did was able to figure out what some who devote tons of time and money and consider themselves audiophiles still do not seem to understand.

The perfect image is only one, small aspect of sound. If you think you need your head in a vise... maybe your head is ALREADY IN A VISE, one made up in your imagination.
@geoffkait ,  you just saturate me with you remarkable humor and wisdom. I just ordered a cryo tank to just try on all my components and media. You think a cryo'd gong will sound better?

Actually you just crack me up.

Seems to me actual real tweaks really are those that make sense. 
Number 1 per responses is speaker location within the room...........for now
Top priorities for me would be speaker alignment, subwoofer placement/integration, and seating position (the later depends on where you get the most even/natural bass response).  Essentially you have to play the room.  Nothing else really matters until you first get the basics done right.  I would highly recommend Jim Smith’s book/DVDs - “Get Better Sound”.  He explains a lot of these concepts in detail.
A pair of Linkwitz Lab speakers
An easy build for $1000
Keep speaker cables as short as possible.

Use shielded twisted pair (one pair for each channel) from your tonearm to phono stage. 

Remove speaker grilles. 
Cryogenics works, at least on metals. Here’s detailed research showing exactly how it profoundly changes metallic grain structure
Another vote for speaker placement.

Some sound better closer to boundaries some don't - it just depends upon the room and what the designer wanted.

For me, it's tweeter aimed at ear level before anything else. Then it's proximity to rear wall, as very few speakers sound best placed flat against it.

@mauriceminor, the ultimate DIY tweak?
In the UK many of us live in older houses which have suspended wooden floors. In these houses bass vibration is a real problem.

The solution is:
1. Put your speakers onto cheap granite/stone slabs. I use two cheap kitchen counter top chopping boards.

2. Remove the spikes from the speakers and replace them with feet that absorb vibration. Isoacoustic Gaia feet are excellent as are Townshend Seismic bars or feet or platforms.

Maybe this also works on solid floors, I have not tested it. Isoacoustic do demos of their feet on solid floors so I suspect it works here too.

Other tweaks that worked for me:

- For the rack sit the spiked rack feet into Townshend Seismic corners. These fully suspend the whole rack so that it ‘floats’. By doing this then I have found that individual component isolation on the rack is no longer necessary.   

- separate AC power circuit for the Hi-Fi, and do not plug any components powered by switch-mode power supplies into these sockets.

- use a network switch between your router and your streamer. Cheap second hand Cisco switches are good, maybe due to good shielded mains power supplies? Although Hi-Fi companies have started manufacturing expensive equivalents Cisco seem as good. Do avoid ‘noisy’ cheap low voltage network switches.

- without wanting to open up a cable debate I will say that if you use usb cable to connect a streamer to a DAC then use as short as possible. It makes a difference.

- and finally, whilst I have not tried expensive Hi-Fi Ethernet cable I can say that Cat 7E is better than Cat 5E. There is good reason as it has better shielding. However I really did not want this to be true as I have Cat 5E cable under the floorboards to my streamer, installed years ago when we renovated the house. But unfortunately the Cat 7E is so much better that I have to have above-floor cables again. Not ideal.

If you get bored and can’t think of anything else to do there’s always the water bowl tweak. Place three medium size bowls filled with ice cold water about a foot or two out in front of the speakers, one bowl in front of each speaker and one between the speakers in line with the other two bowls.

Free tweak for the first person to correctly explain how the water bowl tweak works.
I wonder why no one mentions Master Setting speakers.  I went to a Denver audio store and they demonstrated master set.  He had his speaker location taped on the floor so after moving them out of position they would go back into sync.  He demonstrated how everything sounds better when you can not detect either right or left speakers.  When he moved the right speaker about a foot out of position, we suddenly had to raise our voices in order to hear each other.  The moment he moved the right position, we were able speak at normal conversational volume levels.  I also noticed how my shoulders dropped.  Everything sounded relaxing and the sound came from between the speakers to create a really nice sounding sound stage.  However, when he demonstrated the REL S3 subwoofer, I was not able to detect a great deal of difference.  I think this is because REL designs their subs to act more like woofers than subwoofers to allow them to play down slightly lower.  This is probably why they blend so well.  However if the specs for the S3 say they play down to 21 Hz at -6 db, they are most likely playing down to maybe 30 Hz.  The problem is if a recording contains frequencies that play down to 14 to 20 Hz, you are not going to hear them.  The trick is finding a subwoofer that doesn't pound you out of the room.
@geoffkait I’m familiar the the rock and bottle test, but not the water bowl tweak. 
Nice stuff! Geoff, dude I tried your tweak but was bowled over, watered down sound from my superbowl system. Broke the center bowl while doing the alignment. One good thing, I did mop the floor it needed it.
My girl friend got so excited by the transparency and wetness of the sound she mopped the floor with me. 
Sleepwalker, kudos on your almost grammatically correct post.
As a great tweak for those who still use an aged HT processor but an newer media player use the players DAC instead of the processors if possible. Simply use the analog outs (if available) connected to analog in on processor and set processors configuration to analog bypass. It's an almost free tweak 
Walker Audio High Definition Links
Golden Sound Acoustic Disks 
Isoacoustics Ghia "feet"
Helmhotz (type) Resonators
Crushed Quartz 
Blu-Tack Putty
Ceramic Ball Bearings (under Corian shelves)
Sound Coat Damping Sheets

All of the above have improved the sound quality of my system 
1. Get your ears cleaned,,, Perhaps checked as well.
2. Near Complete Darkness when listening
3. Don't wear spectacles/contacts
4. Eliminate other noise.. Fridges, Hard Drives etc,,,
5. Good Bottle of Wine
6. Music sounds better later at night.

Obtain exact level of the CD to reduce tendency of the disc to wobble whilst playing, which is not good for quality of laser reading, especially in light of the fact CDs are frequently out of round. However, the level of the top of the CD player may not be the same level as the CD. What to Do? What To Do? 🤔
My tweaks
(1) Isolate anything that has moving parts - Turntable, CD player,
(2) Isolate anything with tubes.
(3) Be fastidious with Speaker and Turntable setup.
(4) Every 3 years or so clean all contacts with Caigs Deoxit.
(5) Make sure your cables are tidy - for example don't have interconnects running in contact with power cables.
(6) Prefer long interconnects with short speaker cable to short interconnects with long speaker cable if possible.
(7) Avoid MIT cables.
(8) Use Dynamat on speaker driver baskets and non visible parts of the turntable.

Be sure to clean all non-audio wall outlets in the house. The noise on audio circuits includes noise on other circuits. You know, things like micro arcing.
Noromance: In an urban setting, removing the cover of a preamp virtually guarantees you will be hearing ham radio instead of your music. That was my experience. 
In a comment to unreceiveddogma post... I borrowed a noise analyser from my dealer. IT showed in numbers from 000 to 999 noise on line and had a speaker to also give info. Well the one component that clearly was creating radio broadcast noise into the AC line was the Audio Research Sp-15 Which btw, has open mesh top and bottom of both preamp proper and the separate power supply. So YES a preamp that is not enclosed will pick up plenty of airborne radio and other interference and incorporate it in the electronics and even back out the AC line.
The green Sound Improvement Disc makes a noticeable improvement in the sound from a CD,DVD, and Blu Ray disc . If you can try trimming said discs with a CD edge trimmer like the Audio Desk Systeme lathe you will hear another level of improvement  Not my opinion alone .
The edge trimmer also seems to slightly help video images , but not enough to make that a reason to buy one . They are over-priced new , IMO . Got mine used .

Most everything else has been mentioned here by others . I did build my own power cables using Oyaide Black Mamba 12 gauge for the equipment ( as short as possible) and Oyaide Tunami 10 gauge from the wall to the balanced AC power supply with gold plated copper connectors all around .
Speaker placement and first reflection points - of course.  Isolating the tube amp was huge for me.  Following many of the Jim Smith/Getting Better Sound recommendations also very helpful, for me the biggest was getting rid of the rack on the front wall - gobsmacked.  Makes sense though, that's the closest first reflection point in one dimension.  Got the Ghia speaker feet yesterday and can't wait to try them this weekend.  

Most surprising - the chair.  On another thread, I was tipped off by a certain pizeo-electric specialist that my prized Ikea Poang chair has very poor acoustic properties.  Trials proved he was right, I have a new chair ordered, but in the meantime, now I can actually feel the resonance vibrations in the chair pad when spls exceed 85-90 dBs, and this is not a good thing, for the SQ or anything else.

Oh, and speaker grills stay on for me.  
couldn't agree more with chi chi's comments. I can't tell you how many great -and very expensive - systems I've heard playing with bright lights on in the room.  Getting distracting lighting out of the way - and that includes any led's on my equipment, which I cover with duct tape -  allows my brain to focus solely on the sound.  I listen at night with the glow of the tubes from my monoblocks, the moonlight, and indirect candlelight and I'm I heaven. My system can "sound" twice as good but listen with harsh light on in the room and it's not worth 2 cents to me. Also agree with eliminating fridge noise.
I agree with several suggestions here including cleaning your ears. I suspect that most here are male and older and thus already losing high frequency hearing.   Ear wax build-up definitely makes this worse.

I also recently tightened the drivers in their mounts and was surprised how loose they had become although the effect on sound was not very obvious.

A number of persons whose opinions I respect have praise cyro treatment of materials including Stax plastic headphone cases. However I am not clear on what effect this is supposed to have on the sound and what physical principle is involved.  I am guessing that it may have something to do with damping characteristics of the material, i.e. it changes the plastic so that it dampens the vibrations going into the surrounding material from the drivers.  Grado claims to have a proprietary polycarbonate for its headphone cases which does this and enhances dynamics and attack and it may be something along this line.

My own take comes from making  fair number of tests of the use of sorbothane on headphones (mostly Stax) and speakers. This work is covered in my postings over a couple of years here

Essentially I am finding that extensive damping, in my case with dense (70 duro), smallish sized pieces, properly glued to surfaces and backed with layers of, in my case, electrical tape to provide "constrained damping" has a massive effect on sound quality, in terms of detail, clarity, dynamics, and with phones stereo separation.

There are several companies working on this problem. Sennheisser has been adding polymer materials as damping for years in their TOL dynamics and I would imagine also  in their electrostatic and noting it in their adds although it seems not to be noticed by the fans of their phones. I am confident that we will see a lot more of this in the future.

The costs of damping phones is generally only a dollar or two (although my glue is quite expensive) .  For speakers you might use $50 or more of sorb. 
Nice comments and suggestions, in particular the suggestion of dynamat on baskets. I had forgotten that one. I'd think it would also be useful on equipment covers (inside) I did the dynamat on a set of speakers I rebuild and there was in fact an immediate audible improvement in those speakers. 
I'll be doing dynamat on my covers soon.
Good stuff commentators! 
@geoffkait , Geoff, I’ll take a stab as to how your water bowl tweak works.
Does the ice cold water in front of the speakers slow the bottom portion of the acoustic waves because the speed of sound is not truly a constant and actually slows relative to cold air temperatures? If this is the case, it would cause the top portion of the wave to break, or curl downward toward the listener, changing the sound. Not unlike how a wave breaks on the shore when the bottom of the wave is slowed by friction as it moves across the sand. Am I close?? What did I win?? :)
Lol! You’ll take a stab at it. An obvious case of plagiarism. I think I know my own words. 🤗
How dare you! Not complete plagiarism. I jazzed it up a bit and added a word here and there. For my prize I’d like your latest version of the Clever Little Clock and four of your Acoustic Resonators. This is fun!
Gotta love Geoff, a most unusual character, loved by few and despised by many.
I thought about it overnight and I’ve decided to gift thecarpathian a photo of the Clever Little Clock.
Sweet! I'll put it in the freezer....
@geoffkait & the carpaithian, sweet, so glad Y'all play so nicely. It don't get better than that.
Now get back on subject.
@elizabeth @unreceivedogma
Noted. Being a tube user forever, I guess the metal cover impacts tube pre/phono amps to a higher degree. I’ve been lucky to not have run into the issue living in the spacious suburbs. But it's a tangible effect whereby the sound opens up as the lid is lifted away just like when you lift a plastic lid off a turntable.