Turntable Power Cord Upgrade

The same applies AC or DC. The main exception being if the DC involves a battery, and if the AC charging the battery is disconnected during play. Because essentially, as I just showed, as long as there is any connection to AC then some of the noise on the AC line is going to get to the DC. The one exception is when a battery is used, and even then only when the battery charger is disconnected from the line during play.  

This isn't guesswork or rumor. This is actual experience running all these different ways.

The sound of a component starts with the power supply - not what comes before it! Physics 101! 

False. 
Only if the power supply is perfect. 
No power supply is perfect.
Whatever noise or fluctuations are on the incoming AC, some of that will always make it through the imperfect power supply. 
Because: no power supply is perfect.
Therefore, the quality of the incoming AC does matter.

Logic 101.

My very first turntable mod was to hard wire a better power cord onto the Basis motor. There was no IEC so it had to be soldered. It was a very inexpensive power cord. Well, I wasn't about to go cutting up an expensive one for an experiment now was I? Yet in spite of that it was a very noticeable improvement. 

If a power cord is good it will be good on whatever you use it with. If not then guess what? Not a good power cord after all. It really is that simple.