It would be interesting to hear the outcome of measuring the voltages of the PSU, particularly the 8-pin PCIE cables that go to your video card. Remember you need to test at the cable end in case the fault is in the cable.
Go here for checking your voltages.
It's a shame you can't get hold of another graphics card and test that, easiest option.
Have you tried testing the system out of the case? Sometimes things short only in certain areas, like standoffs and rouge screws underneath the motherboard.
Assuming that the companies that you RMA'd the components tested them correctly (did the mobo manufacturer really install a graphics card to test it?) then we need to eliminate them from the faulting, so:
Motherboard - presumed okay (RMA return)
Graphics card - presumed okay (RMA return)
PSU - not completely tested
RAM - unlikely candidate
CPU - extremely outside chance this is at fault but not unheard of - perhaps worth checking for bent pins on the motherboard
If your PSU has good voltages then I'd suspect the motherboard again and that ASUS were lazy, incompetent or both when testing it.
Edit - as both your motherboard and GPU are ASUS, how about sending both at the same time for testing? Only after you've checked your PSU though

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