Not sure what you mean by "
ONE of us should have thought of that."
I did. That's why I looked at your QVLs and noted the 7670 is not listed. But you said the 6400K didn't work either and that is listed. And according to TPU
here, that AMD processor was released in June 20
13. Then I looked at the board's BIOS history and according to
Biostar, the initial BIOS release was in Oct 20
14. So the 6400K should have worked with the board's factory release BIOS.
It is possible those boards sat on the retailer's shelf or in their warehouse quite sometime after they left the factory before you purchased them. But the information you provided in your opening post, along with my findings about the CPU and motherboard release dates, did not suggest a BIOS firmware update was needed.
I also thought you might have zapped the CPUs with ESD but you said they worked with other boards so I did not mention that. But it is possible you zapped the boards if you did not take the necessary ESD precautions.
The extra standoff(s) issue is actually a pretty common mistake by newbies and distracted pros alike - especially when swapping boards out of a case that's already been used in a build with a different board. This is because cases are designed to support 1000s of different motherboards so there almost always are more standoff mounting holes than motherboard mounting holes.