Your most indispensable "tweak"?

At first, I thought it was the Niagara 1200 but the Isoacoustic Oreas turned out to be more than icing on the cake. I'll have to give them equal billing.

All the best,
Nonoise

Everything matters...I've been tweeting for years.  My last one gave me such an enormous change, it was like changing a major component.  I replaced regular plastic cover outlet plates to Carbon Fiber.  ..a huge difference.

I have a CD issued by high-end solid-state amplifier company Ayer Acoustics titled "Irrational, but Efficacious System Enhancement Disc". My dealer, now out of business(owner died), let me borrow it. Never returned it. It has white, pink, and 3 brown noise tracks. Brown noise is ideal for breaking in everything that needs breaking in: Cables, speakers, amps, DAC’s, etc. The last track is a 5 minute glide track that goes from 5Hz to 20kHz. Played at a moderate level, it makes your system sound better. It’s available at Ayre’s website. I’ve been using this disc for 20 years. I don't consider AC filtering components tweaks. Not anymore. Every reviewer in The Absolute Sound and Stereophile lists AC components in their gear. I have a long discontinued PS Audio Duet passive filtering device. Their AC Regenerators are bad ass, but Michael Fremer, a reviewer I respect, told me "I'm not plugged a power amp into a power amp." Audioquest has a good article on AC treatment. Of course the writer pushes his own design.

Hands down, Star Sound Sistrum SP 101 speaker platforms.

For me, the earliest and cheapest was adding a dedicated line (took one afternoon, less than $100). The most recent, and maybe best, was Krissy's boxes from former PPT: game changing. In between, early-1960 6085 and 7062 tubes (in place of 12ATx and AUxs) for the line stage of a LTA Zotl-10, which itself was a total ear opener. I've also been happily surprised with the SR HFT/FEQ treatment and IsoAcoustic Gaias. All in all, I've been amazed how much tweaks really do matter... I used to shake my head at my brother's Cryo treatments, but now I have an open mind: if I hear it, I go with it. I listen to single-driver speakers with Fostex F166e drivers (4.5") and it sounds like a concert hall.