Test a new dedicated circuit before installing it.

I've tested my voltage over the course of a day and it was 123v constant. No fluctuation at all. 

So, if I pigtailed to a SB receptacle would it still be beneficial using that existing circuit and coupled with the Niagara 1200's? The noise floor, to my untrained ears is pretty low, but I do hear a slight hiss from 10 inches away from the tweeters. 

I'm trying to do the "can't really hurt" little things while waiting to upgrade other components. 

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.

First, I'm here to ask questions because I don't know the answers. I'm going to have to re-read this thread more carefully to fully absorb it. I'm not sure if all the answers were agreeing. So I would have to decide who is right and who is wrong. More reading. 

I already purchased the tester. I'm not going to return it. No big deal. I saw someone demonstrate it on the Niagara 1200 and thought it was interesting, so I bought it. I figured I would see what my numbers were before and after, and possibly after a 10 gauge cable run... I wish I would have asked before the purchase. 
Thank you for the info about the hiss at the tweeter. I asked and you answered. I am of the belief that the more info the better when asking questions. That's why I mentioned all the above. Sometimes it's important. Writing facts about the rest of my system may have come across as argumentative or defiant, but it was just adding more info to my questions so one can get a better understanding of the situation. 

Thanks again, I'll just keep reading.

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.