It’s not hard to make the comparison between 1 leg and 2 when you have gone through multiple systems and step by step made 1 change then listened. It makes a difference. And anyone that is caught up on just focusing on the phase line and voltage that is regulated by the equipment power supply is missing everything going on with the neutral and ground and how those 2 play into the system.
Again, you can not hide from power line noise by selecting a phase. The noise is on the neutral. Unless the noise is from a 240 volt motor, in which case, it’s on both phase, so again you can not hide from it.
Yes the power supply in the equipment regulates the incoming 120 volts to something else. But most equipment also has some connection of the equipment power supply of the signal ground that is tied to the chassis ground. So the signal is tied to the 120 volt power grid. I have heard it. Others have heard it. Just do it. In my opinion, there is no reason to not use 1 phase. There is 0 evidence only 1 phase causes any issues. So why fight it? To make a point? If that is the only reason you are arguing, your helping nothing.
And, people here seem to think it’s great to put metal clad cable in and tie the ground to the shield of the cable which then allows the shield to become an antenna. Or at a minimum you have introduced a ground loop because the impedance of the shield and the copper wire are different. And before some "Engineer" gets on a rampage, let me say I have removed more than one annoyingly loud ground loop by eliminating 3 wire MC and replacing it with Romex.
I don't have an engineering degree, so I don't talk as technically proficient as others. What I have done is consult with many engineers, nuclear and NASA scientist. I have read a lot. I have taken specialized classes on grounding. I have made alterations, observed the result, then sought out knowledge from these type people to understand why I heard what I heard. I have also wired maybe 100 systems and have a large data base of evidence to back what works and what does not. I will say again, putting MC into an electrical supply is a receipt for disaster unless you are very cautious. IF you only ran 3 wire MC with a hot, neutral, ground and metal shield, you’re likely to have noise and/or ground issues. If you ran 4 wire and don't use an isolated duplex on even 1 outlet, you may introduce ground and noise issues. Even 4 wire MC is not a full proof way to eliminate noise. The jacket is an antenna for RF. It really does nothing for audio performance. Commercial facilities use metal for fire protection. Not because it has better performance characteristics. Just like your house has AFCI on all the breakers. They don't make your computer work faster or stereo sound better. To that matter, I have sat and listened with others and you can hear an AFCI breaker as a small veil on the music. They hinder audio playback. But they extinguish arc fires before they propagate to the rest of the house and burn the building down. They do a great job at that. As do metal raceways. I have been in a room when the switch was thrown and a faulty piece of MC arched inside the cable jacket. It was a very large boom and smoke smell. It took a while to find the fault as the case was fully intact. Just a small discoloration of the jacket under the flash. No flame left the jacket where it could ignite combustible materials around it. That is why you use metal jackets. It’s fantastic fire protection.
The only shielding that blocks RF to any degree is a copper pipe. No one runs wire in copper pipe. Steel and aluminum don't really do anything towards blocking RF. They do block EMF. But cable management eliminates issues associated with EMF radiating from your power cable and contaminating other circuits in close proximity. That's all metal is doing for you. Maybe you want to run the rest of the house wiring that is in close proximity to your audio outlets in metal. That is a good solution.
The only real block to RF is putting something such as a Torus Isolation transformer on your rack. The best location for a isolation transformer is on the rack. The second best is within 10 to 15 feet of the rack. After 40 feet, you have potentially lost much of the RF noise mitigation as it has reentered the circuit again. Metal won't keep it out. You can coat a room with Faraday shield. But it’s very hard to do correctly. I have heard of one audio room I want to visit and been in a commercial room that was RF treated. The one I was in was a government building where they wanted to house sensitive equipment. My phone still partly worked. It was very intermittent and definitely impacted it. But it was not 100%. I would like to get in the audio room with my RF sniffer and see what is still there. RF is an interesting phenomena. In my room, my meter reads about 1700 near my rack. Even with the equipment turned off. It’s not my audio equipment. Walk 24 feet out of the room and into my hallway and it reads about 70. It’s a massive drop in only a few feet. Directly above my audio room in a bed room the RF is around 1700. Something about that side of the house.
I just came back from a work trip where a client was crying about a radio station bleeding through his speakers. I was confident it was not my power supply, but I had to go check. In the end through various processes of elimination an use of filtration, I was able to validate his system preamp was a mess of noise and his phono preamp was not filtering out the RF from the cartridge, tone arm, phono pre enclosure. It is my experience through many people asking for my assistance to eliminate some sort of noise, I find the issue is the audio equipment itself that is the culprit. I have seen it with 3 year old 40K monoblocks to brand new $20K phono stages. I have never seen a clients noise issues emanate from a well designed electrical infrastructure using NM cable. Having said that, I have dropped Torus Isolation transformers on peoples racks and 2/3rds of people end up buying it. I sold 2 out of 3 on this last trip. There is a ton of noise on the power line. But it’s not noise you notice as a hum or radio. It’s a hazy veil most people don't notice. They just keep buying new gear trying to get better performance. Once you eliminate the power line noise, the music becomes more clean and quiet. Not black. The room feels more calm. Details are more present in the recordings and the bass becomes tighter and more integrated with the whole. The music is more natural. There is noise on your power line. Lots of it. But you’re doing nothing to get rid of it by wrapping a few feet of your infrastructure in steel or aluminum. Those that say shielded wire does not matter because there are hundreds of miles of wire that are exposed to noise injection have a very valid point.
FWIW, that radio station bleeding out my clients speakers. You won’t fix that with an isolation transformer. You won’t fix it with any power line filter. You might fix it with about $25K in Faraday shielding around the room. That includes over windows, in doors, under the floor. Everywhere. You have to encapsulate the entire room. Or the entire rack. It’s possible a system set in a closed cabinet that is completely shielded will work. And then it’s a maybe. Better to get audio equipment that has a power supply designed to reject RF and not modulate it into the power supply of the audio equipment.
Here is a good read. More important, go to the References and read all of those. I have read a lot of the materials noted in the references.
In summation, if you want the best power for your audio
Use all copper panels and wiring. This means the neutral and ground too.
Use an isolation transformer
Use NM #10. Better to use twisted wire. Better to use grain oriented twisted wire.
If your dead set on using metal encased wire, use an isolated ground system
This is a very high level list. It lack all detail on how to set the infrastructure up properly. It’s basically a one line.