So basically you have a new mobo, new cpu and tested all the parts, also gone back to basics and just left GPU, CPU, ram in and it still wont work...
hmm..
I would say check you plugged everything in right espcially the 4 pin connector to the mobo, also the power switch is on the right pins etc.
if 2 mobos dont work then it must be a connector missing you plugged the PCI-E connector to the graphics card?
but if it worked and then the next day it doesnt...its just odd.
I must say think iv run out of ideas...
OR the actual power cable from the plug to your power supply is faulty? lol its a long shot just throwing anything at it now...