Your most indispensable "tweak"?

Trying to narrow my possibilities down... 
George, do you also have PPT "Gate?"
Do you use them together?  What is the strength
of the "box?"  How does it work?
nonoise, Where is the most noticeble impact of the "oreo", turntable, preamp amp, CD player ?
nonoise, I meant: Iso acoustic  Orea, sorry .

i used 4 oreos as isolating feet under my dac and tt

it definitely sweetened the sound... more than a little  ...  ;)
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. 
Alien Tweak Audio