How To Do You Measure the Quality of Your AC Power?

I use PS Audio products which can measure the amount of incoming THD which is usually in the 3% plus range for me. After regeneration, the THD drops to the -0% range. I feel that that method of measurement and subsequent cleaning of power is beneficial to the overall sound quality. 
Additionally, I have installed Environmental Potentials EP-2050 power line industrial EMI/RFI/spike filter. These are used in massive server farms, IT systems, medical electrical systems, large corporations where critical systems rely on flatline clean power delivery. 

On cables, especially newer Shunyata with DCT to help with noise, hmmm, they are able to lower noise floors below the already impressive -0% noise floor. Can we even hear that? I mean really, can we hear that level of noise reduction or is it already below the perceptible hearing level that would make a difference. What  does a .0001 noise floor sound compared to a .001 noise floor? Can you hear it? Power cables are part of recipes…many in the community hear differences in what power cables do, myself included. Heck, my stock cables sound different, and better in some positions in my system than other power cables. Ahhh, different….what’s that? Noise floor, hmmm. Did that new cable lower the noise floor and I heard it, or, did it change the perceptible audio spectrum which alludes to more recessed mids, increased bass, and sizzling higher frequencies. Look, power cables are like ingredients in a recipe. They can alter the soundscape, but, be careful what you ask for. Power delivery by any cable to your system by default is going to be great. When changing out power cables, all you are doing is changing the ingredients in the power delivery chain which may or may not change the flavor of your sound. Trust me, I just splurged on a half dozen Shunyata Sigma NR 2s and experimented with different power configurations and each component reacts differently to power cables, sometimes to the detriment of sound quality. Get your ingredients right and in the right order. Another example of new Sigma NR2s in my system…my volume must be turned up at least 1/4” higher to get the same loudness level as before. So much for instantaneous power delivery of cables. My system already jumps out at you with instant speed and can play loud as heck…but, power cables alter something, maybe the background is quieter, but why? Ahhh, my highs are slightly veiled and lost some organic sound qualities. Switch the ingredients, maybe new cable break in. Another cable ingredient to consider. 
 I agree with other posters about your electronics and amps having the ability to cope with power anomalies to a certain degree which still results in awesome musical enjoyment. 
Methodically evaluate what you do and do it in small steps. Sometimes, the little things are the biggest payback in musical enjoyment. 

Most audio components are completely immuned to dirty mains power. There are some exceptions (some Rogue Audio stuff for example), but otherwise, the most common problem is humming/buzzing toroidal transformers from DC offset. This is something your power provider can check for with a special meter. They should do that free of charge if you have a legitimate concern. 

https://youtu.be/OL23xkLfPTU

If dirty mains is an issue, it’s best to just stick with the bigger name brands that have competent engineering chops. 

As mentioned many times above (very good advice) it seems like you have a voltage drop problem for whatever reason. Please pursue this with the appropriate pros as detailed above very well.  

If you are curious to "see" what's happening for yourself you can purchase this at Amazon for ~$28.  It will read out true RMS voltage continuously.
https://www.amazon.com/Poniie-PN2000-Electricity-Electrical-Consumption/dp/B0777H8MS8/ref=sr_1_3?cri...

I find mine very useful in the watt readout with individual pieces plugged into it.  Simply to see start start-up wattage versus idle wattage.

Plug this into the wall and plug the treadmill into it. 

This suggestion in no way negates your need to find a true cause.

Regards,
barts

I'll add in my $0.02 for what it's worth on the subject.  To answer the OPs question I don't concern myself with EPQ measurements of any significance.  For me, conditioning the incoming power is a very basic requirement to cleanup the harmonics and spikes you receive at the house.  I use legacy Monster brand conditioners on my primary and secondary audio systems.  These read a constant 120.1 vac all day long, and I'm good with that...condition it, fahgettaboutit! 

Is your incoming dirty?  Absolutely.  Is it a pure sine wave and distortion free?  Ha, no way!  Here's why. 

Power starts with a turbine/generator at your local station, leaves the building at 23 kv or so at a rock solid 60 Hertz.  From there it feeds numerous auxiliary/station service transformers (tx) and hundreds of motors back inside the plant.  When it leaves the building it hits generator step up tx units taking it typically to 230 kv, then 500 kv in the switch yard for transmission. 

Every tx adds in a bit of influence, and harmonic distortion especially in the 1st and 3rd orders up to the 21st order or so.  New tx included,  but even more so with aged units you also have internal partial discharge and external corona over a wide band of frequency ranges . IEEE/IEC standards dictate what level is acceptable for the particular vintage, class, kv and size of the tx .  Anything over 21 kv to ground will go into air corona unless shields are installed. Count in dozens of switchyard breakers, each with internal and external connections, as well as bushings, connectors, buss conductors, tap changers, wire, terminations, lightning arresters, insulators, switches, ground connections, cap and inductor banks, so forth and so on.

Leaving the switchyard down the transmission corridors the feed travels for miles; in my state there are over 30,000 miles of transmission and many hundreds of feeder substations to step down the power for distribution at low voltage, 25/14.4 kv and12/7.2 kv are common.  In these stations... more switches, breakers, tx, connections, etc. and miles of wire on the street.  I`m lucky, the closest generating station is < 20 miles from the house, and the closest substation is <6 miles away.

Regulators (regulating tx) constantly jog the distribution lines based on loading to keep voltages at your neighborhood, when stepped down by those little green boxes (or grey garbage cans on the pole) close to 240/120 volts.  These can be set to narrow or wide tolerances but keep the incoming between 115-125 vac at the kitchen outlet.  Is that dirty? Yes indeed.  Will you ever receive a pure sine wave off the street? Nah.

If you hang an Agilent or other such really high Q analyzer on your lines you would likely see flat tops and transient spikes in the voltage trace.  Harmonic distortion in the current trace will look more like a sawtooth than a sine wave.  Google spends millions on power conditioning at their data centers. As an audio aficionado I spend a little.

Every large utility will have an EPQ group, 8 or 10 guys and gals to investigate significant power quality issues.  Hospitals and industrials come first followed by commercial, the lonely chicken rancher who has
5 or 10 acres and 10,000 chickens that wont lay eggs, then Mr. & Mrs. Residential.  If you have a critical medical device in the home you just got upgraded.  Reach out, be cordial, ask for help.  It's what they do.

Most significant issues will likely be found in the ground connections. 

I hope I've added some value to the discussion.  Every time I flip a light switch, and the light comes on, I just smile a little bit.  Freaking miracle that is!