What is the best tweak?

Got to agree with Onhwy61. Positioning is number one. Consider all the angles of your speaker orientation, do they aim up down or straight ahead, toed in or straight, different heights if possible. Try them farther from and closer to the wall than you might think, especially if you have a sub. Position yourself, be aware of reflections from sides, back and coffee table especially where the sound travel path will have a difference of less than 5 ft. Try the differend phase switch positions on your sub. Then, in two months or so, you can worry about the things that cost money.

Dweller- you should also use you cleaning treatment on the main ground connection to your home. Clean and polish the ground wire, clamp and ground rod.

I use a high speed 3M buffing wheel mounted on a bench grinder to deal with AC connectors and spade lugs.

Kana813: The buffer will make the plug flanges look better but the flat file will make them uniformly flat for more contact. I've filed down many plugs that looked flat but had tiny hills and ridges on them. Good idea about the grounding rod. How often is this necessary?

Dweller- My home is 15 years old, and this year was the first time I cleaned the grounding contacts. I guess it depends on where you live and the water/salt content in the air. I treated contact area with Caig R5 Power Booster(try some of this on your plugs), so maybe I'm good for another 15 years.

PS- before you disconnect the ground clamp for cleaning, run a jumper/bypass connection, just to be on the safe side.

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