Your most indispensable "tweak"?


Trying to narrow my possibilities down... 
redwoodaudio
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.  
Shunyata Denali V2 with Sigma XC power cable.  Made a believer out of me!