Your most indispensable "tweak"?

Many tweaks depend on what you want to tweak. before you say over all sound and as some one mentioned divorce comes into play or refinancing your home. Simple tweaks that wont break your  bank account. For instance TTs, there are many tweaks for TTs, that we can discuss.
 Speakers, placement comes into play. Place a laser on top of your speakers see where it points at. This may help in re-directing where they should pointing or placed, if the laser hits your sweet spot too low or too high adjust them to near ear level. Next you may want to go after internal wiring or capacitors in your crossovers. a better cap makes a big difference. Believe it or not even expensive speaker companies don't use top quality caps. We've changed out caps and better wire in less expensive speakers and made a really big difference.
 
 Speaker wires make a big difference, however this could go either way good or bad. More expensive doesn't necessarily mean better. 
This  depends on how much noise the speaker cable lets in, meaning it works like an antenna. Connect it to a FM radio antenna connection  with a tuning meter and move it around, get a high reading on the tuner meter, not a good choice.  braided or twisted pairs works better than most. the less noise or interference, the blacker the background.   You don't need huge snake size cables either. A simple tweak raise them off the floor.
carpet tends to smear the sound. I'll bet your cables are on the floor. if possible away from the wall as well. small pieces  foam works well for testing. You'll notice a difference.  
Try good power cables on each of your components. and route them accordingly. away from your speaker cables and interconnects if possible. Which is next to impossible.. 
Clean your phono jacs and connections with pro- gold. little goes a long way. spray it on a cotton swab and clean all your connections. 

These are just that tweaks.. Wont break the bank or your happy home. 
GT
Alien Tweak Audio

All tweeks build on the tweeks before....sometimes a wonderful tweek doesn't work with a previous wonderful one.....a never ending search....

Hi redwoodaudio

I could not agree with bdp24 more. Room treatments are often overlooked due mainly to the  wife acceptance factor. Let's face it, most audiophiles don't have the luxury of a dedicated room. If you do, don't underestimate the benefit of room treatment. A word of caution, it's very easy to over do it. When done properly it can make the difference between good sounding system and a great system. 

Placement of your speakers is absolutely critical. The difference of an 1/8" can also be the difference between good and great sound.  

As far as tweaks are concerned regarding best bang for the buck, I think a good set of isolation feet can sound amazing (if you get good ones). I bought a set of the Critical Mass System Center Stage feet and couldn't believe the improvement. They are stupid expensive but boy do the work. In my opinion, they are worth every penny. I've tried just about all of them over the years with very different results. Some were good, some were just ok, some made it sound worse. You have to be careful. At first you think everything is better and then you remove them after a few weeks and you realize that it just made things sound different, not necessarily better. I had one set I thought were great and when I removed them and put the cd player back on just the shelf in the rack my tonal color came back. When they were under the cd player the tonality and timbre was gone! Caveat emptor. My advise is avoid coming to a quick conclusion. Take your time and listen carefully and then remove them and listen to the same material. The CMS feet really just blew me away. The new Wilson "Pedestal) feet are also supposed to be fantastic.  Hope that helps. Take care.

Scot 

Got serious about cords and clean power.

Closing the curtains when listening. 

Tweaks that worked for me:
- Townshend seismic corners under the rack. These ‘float’ the whole rack and made a big difference
- cheap Granite slabs under speakers
- Isoacoustic Gaia feet under speakers
- CAD GC1 and GC3 passive grounding units
- a dedicated Hi-Fi ring from main fuse box- closing the curtains (eliminates reflections from the glass)

Tweaks that made NO difference to me:
- Isolation feet under individual components (and I tried three brands just to be sure) made no difference. Maybe because the whole rack is floated on Townshend seismic corners.
- power conditioning, even a top of the range £6k conditioner. It smoothed the sound but did not improve it.