Your most indispensable "tweak"?

Trying to narrow my possibilities down... 
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.

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,
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.