Test a new dedicated circuit before installing it.

You won't find much that is worth reading. The vast majority of what's written is either by electricians trying to help people who know nothing wire a house, or by audiophiles who read the stuff written for people who know nothing about wiring and so now they think they know it all. Almost none of what you will find is written by guys with real hands-on experience actually doing this and listening to the results. That would be me. Most everyone else is clogging up the interwebs repeating what they read somewhere. Or they paid some electrician to do something and so now they are repeating what the electrician told them. On and on.  

The reality is in terms of money one 20A wire straight to the outlet is a huge upgrade and hard to beat. But not for any of the reasons you are likely to read about. Also yes every single tiny little thing you do will indeed make things either better or worse. I have put little dabs of contact enhancer on my circuit breakers and the improvement is just as great as if the same dab was used on the speaker terminals. Not conjecture. Did it. Heard it. 

I am a very value oriented big picture audiophile. My obsession with awesome sound is about a million times greater than my income. The reality is that for every one thing you do that's one less hour or dollar you could have done somewhere else. In the big picture, bottom line, you said "AV setup". As long as that is the case do yourself a favor, run the one line, and don't waste a single second more on power. You have bigger fish to fry.

The fluctuations you're measuring are irrelevant. This is for a music system. Music is constantly fluctuating, causing the amp to draw fluctuating current, causing line voltage to fluctuate. Not drop, not anything you can measure. Honestly, if you are going to run around measuring instead of accepting the experience of those who have actually done this stuff what is the point? 

The noise you are hearing with your ear to the tweeter is NOTHING to do with AC line noise. That hiss is normal and comes from the components themselves. You could run them off batteries and still hear the same hiss. So far off track here can't even begin to tell you. 

If you actually do want to improve AC cost-effectively run one direct 20A circuit, period.

Nope. Not even. Worse than a waste of time, whatever you get from a noise meter (extremely questionable to begin with) becomes totally worthless when you realize a lot of the noise comes from where the wire is- and you're not testing it where its going to be!  

My system https://systems.audiogon.com/systems/8367 has been in this room nearly 30 years now. In that time its been run on a normal circuit, a direct (dedicated) circuit of 4ga wire, a direct circuit of cryogenically treated 4ga wire (yeah, I pulled the wire out and had it cryo'd!) and that's just the 120V history.

Then I got a step down transformer and so now its cryo'd 4ga 240V to the transformer. The wire since then hasn't changed but there's been a ton of improvements, the whole panel and wire being treated with TC and some other stuff. 

Each of these steps was an improvement but the biggest most cost-effective one was the first, going direct, eliminating the daisy-chain of outlets. For what you are talking about its probably worth running one dedicated line. Make sure it goes close to your system, use a Synergistic Blue or Orange outlet, and call it good. 

By the way: use your ears not a meter. Every time.