You can only benefit from one type of armor at a time
if you were somehow able to wear two suits of armor at the same time, only 1 calculation for your AC can be used, so I would assume you pick the higher one
I would also like to point out that under the power armor ability
The armor continues to be power armor until you doff it, you don another suit of armor, or you die.
So donning a suit of armor over top of your power armor is going to cause your power armor to not function.
To clarify on your bounty. The magical bonus granted by magic armor is included as part of the armors ac calculation.
For example breastplate is 15+dex(max 2)
A +3 breastplate is 18+dex(max 2)
+3 plate armor is 21
DMG pg. 140-141
Using a magic item’s properties might mean wearing or wielding it. A magic item meant to be worn must be donned in the intended fashion: boots go on the feet, gloves on the hands, hats and helmets on the head, and rings on the finger. Magic armor must be donned, a Shield strapped to the arm, a cloak fastened about the shoulders. A weapon must be held.
Use Common sense to determine whether more than one of a given kind of magic item can be worn. A character can’t normally wear more than one pair of footwear, one pair of gloves or gauntlets, one pair of bracers, one suit of armor, one item of headwear, and one cloak. You can make exceptions; a character might be able to wear a circlet under a helmet, for example, or to layer two cloaks.
Armor isn't intended to be layered therefor stacking magic armors doesn't provide magical benifit.