Your most indispensable "tweak"?

Best of all:

2 dedicated audio lines with hospital grade outlets and 10 gauge wire. Was not expecting it to make a big difference...and it wasn't...but it wasn't a small difference either. It was a solid upgrade.

I had ordered a new tube amp when the lines were put in and I almost wanted to cancel.
I had a Carver with great high end but weak bass...and the bass was now sublime...overall sound was more focused.

CS2 footers
Orange fuses from SR

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.