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

What a freaking waste of time and money. None of these people will find anything wrong, because from their point of view there is nothing wrong.

Multiple electricians have failed to find anything wrong with my electrical system or electrical panel. 

Exactly. There is nothing wrong. Sorry, but there just isn't.  

Only thing wrong is your approach. You heard a difference and assume this means there is some glaring fault somewhere to be fixed. There isn't. Instead there is just the normal problem of noise coming into the line.   

The kind of noise we are talking about is endemic, and involves everything from RFI from radio, cell phones, and appliances, to EMF from everything connected to power everywhere.  

It is what it is. What you do is what everyone does. Make all your connections from the panel to your room as clean and tight AND FEW as possible. This is where the "dedicated" (which really means direct) line comes in. Then from there you use as good quality outlet, power cords, and conditioner as you can.   

The better the quality, the better the results. Results which are by the way determined by ear, not by meter. Unless your system goal is to sit in the dark looking at a meter?

Maybe, but maybe not. What I have noticed about electricians and electrical engineers is that they approach it from the perspective of meeting code and not starting fires. That’s not a bad thing in general, but if you’re chasing high fidelity sound you’re going to have a bad time with that approach (minus the not starting fires goal, haha!)

If code allows for a 5% voltage drop in a 120V line, then you’ll see fluctuations between 114 volts and 120 volts depending on the load. That’s completely unacceptable for hi-fidelity sound. You want that voltage sitting at 120 constant and not budging an inch. Sure, a good power supply will minimize the effects of voltage fluctuations but why would you want to deal with that hurdle if you don’t have to? Make it as easy as possible for your equipment to perform, which means going well beyond what a code book would tell you. 

Your advice with a dedicated line is spot on, however, it won’t fix problems upstream of your panel such as a loose lug on the meter socket. There are other things going on outside of my audio system that strongly indicates a poor ground or bad neutral connection.

I could pull a dedicated line, I could buy a filter, I could buy a regenerator, and I probably will some day, but there has to be a better method than “just try whatever and see what helps.” If you can diagnose the problem properly, then you can apply focused solutions which saves time, money and headaches.

Early in 2020, on the recommendation of my electrician and @jea48, I contacted our Power Company and requested a power quality analysis. They installed a special meter and downloaded weekly readings for a month. It resulted in some repairs to our service, and to neighbors’ service. One repair improved voltage consistency. 

It was free. Well worth investigating.

That’s exactly the type of thing I wanted to hear about, tvad. Thank you!