Test a new dedicated circuit before installing it.

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.

The only benefit of 10/2 vs. 12/2 is lower voltage drop, which increases with current.

You are unlikely to notice this unless you have a long run. Certainly it won't reduce your noise.

I have about 6 receptacles on the current circuit, but only the AV system is using it. Using 2 of the receptacles. They are daisychained like MC mentioned. I guess connecting the wires together and pigtailed off them onto the receptacle would be better than going through the receptacles to finish the circuit.

The intention was to install one receptacle and plug in 2 x Niagara 1200 units to that receptacle.  Then, everything would plug into them.

I'm lazy enough to just leave it as is and just plug what I have now,  then plug  into the 2 Niagaras.

I already ordered the tester, so I'm sure I'll Tinker with it.

Thanks for the replies. 

My panel is < ten feet away from my system and I have some access to the area. But again, lazy...

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.

MC, I've seen your posts against HT receivers. We all have our limits i guess.  My goal is to get an uber(to me) integrated amp with HT bypass. I absolutely love my setup right now. Its not broke, but I want better 2 channel. While waiting to decide, I figured I would add these extras like speaker cable, interconnects, new circuit, receptacle, conditioner, gaia feet,... maybe all if them cumulatively would make a difference when I do change equipment.

I'm still deciding between integrated and get rid of my Anthem str amp, or preamp and keep the amp.

I'm getting there. It will never be ideal because some things can't be changed, like an 80 inch tv right between the speakers, and furniture, coffee table etc etc... but we do what we can.

Thanks again for your time.

I just ran a 10-2 romex to the panel. Not bad for an existing wall, putting in an old work box and ran into the panel in less than an hour.  Total cost <$40.  Can't go wrong with that in terms of time and money. Let's see if I hear a difference. Nah, not yet, I'm still upgrading to make a difference I think. The only thing I will probably change is the receptacle. I just got one to finish the job for now. It's a heavy duty commercial grade or something. It has somewhat of a good grip on the plugs and that's what I wanted for now. 

The Niagara 1200 is coming in 2 days. Will be putting that into use.

Thanks again for all the guidance.