What is the best tweak?

Each one of your listed tweaks has had a major effect on my system. My Golden tympanics think that they hear a difference with the Cryoed Hubbell outlets. That may be the power of suggestion, for me, but the placebic effect (having a wife, who is a pharmaceutical rep, helps) may be what I'm really hearing. That little tweak, as well as cleaning posts etc. are an accumulative thing. I'm a nut job with electricity tweaking. Electrical properties and how they affect sound are quite a mysterious thing. I am going on my third power conditioner. I am waiting on delivery of my new Audio Magic Eclipse. My Stealth, and then my Matrix have been wonderful additions. In fact each one of those things on your list had major influences on my system. Perhaps, because I'm waiting on this new electrical wonder, that's what's on my mind, most. After break in, stay tuned for my thoughts. I, as well as all of the audiophools out there, can ramble on (quite cogently) about any one of those things on your list. I don't think of them as tweaks. They are major components of a sound system. Cleaning posts? Cryoed outlets? Spades rather than banana clips? Those are tweaks. peace, warren

Viva la differance, I now own 3, two that work and one for parts just in case one breaks.

If you own vinyl and can find one, buy it.

loon

Here's a general answer to a general question. Pull your speakers at least three feet from their rear wall and put bookshelves(with books) or a cloth curtain behind them. Put large potted plants with small leaves at the first reflection points and a tapestry behind your listening couch.

What is the best part of mathematic to know?

Alternative: skip all $$$ tweaks: No $1,000. cables, no $700 powercords, No $300 jars of rocks... no $200 cones...
Save all that money, and just get better primary equipment in the first place.
With better equipment, you can skip the super tweaks... in fact you can use cheaper interconnects and power cords, and STILL HAVE BETTER SOUND, than with lesser equipment, and all the tweaks in the world.
Yes I am saying, for the money spent, a better grade of basic gear is worth WAY more, than a lesser grade of gear with money thrown at a pile of tweaks.
My (humble) $0.02/experience from 38 years of audio insanity