Your most indispensable "tweak"?

My tweaks involve a soldering iron and only the amplifier.

1. Replace the Schottky rectifiers in my amplifier's power supply with SiC rectifiers.
2. Replace the Mundorf interstage capacitors with Jupiter copper foil caps.
3. Replace the volume "potentiometer" with a stepped attenuator.
4. Replace the stock output transformers with Lundahl transformers.

The  audiodharma Cable Cooker is well worth the cost. Really improves my cables and also gets rid of the break in period for new wires. Perfect when I get back from vacation as well.
If you want to see if a new cable is better, just cook it and you know right away.

Of course, the real tweak is Super Audio CD.  The extra cost for a SACD over a CD is tiny compared to the improvement in sound.

@testpilot 100% agree.  Unless cabling is considered equipment and not tweaks.  In that case, Hallographs and SR HFTs are most cost effective and necessary acoustical room treatments.

Oh, and Stillpoints under my equipment, especially my pre-amp which is custom made but had a dark, slow and mushy sound on it’s hard rubber cone feet. Townsend seismic sink under my VPI TNT turntable-essential due to VPI’s poor vibration control.  My room construction is not a tweak but considered an essential component for music reproduction.  It was expensive, non-defeatable and provides both perfect isolation and bass control.

+1 for High Fidelity.  MI started with the 0.5 and upgraded twice. Not cheap by any means when you start getting into the 1.0 and up, but I just love the clarity it brings. BIG improvement.