When the hum does come, or at least right now, it’s from the right ATC’s bass driver and the left sub. It can also be heard during very soft passages. But then at a different moment it will be from the main speakers only. I suspect if the refrigerator condenser is running or not affects the hum state. In any case, turning off the dac seems to kill it.
The sound my Apex makes when I power it up sounds like it’s coming from the furnace, 25 feet away and behind two doors. Those capacitors filling is weird sounding. But then it stops as does any sound. Weird. When I plugged it in to the P20 it made odd and sad noises.
But DC in an AC line. I would expect the P20 takes care of any trouble with that being taken out in the AC to DC conversion. It doesn’t make sense to me that any DC issues would exist in the DC stage. Wouldn’t that be the same with a P15? I can’t imagine a need for something else.
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Logical 
PS Audio’s write up doesn’t say anything about DC and Paul says the regenerators are only 80-90% effective.