X11SRA-F died?

I have a server that after 2 1/2 years of smooth operation does "nothing". No fans, no beeps.
I've stripped the board down to the processor and fan, and out-of-the-case, grounding the front panel PWR-ON connector does nothing. Not even a short twitch of a fan.

If I ground PS-ON on the power connector, the fans do run, although I don't run the experiment very long since it's not really a clever thing to do.

A spare power supply doesn't help. I don't have a spare processor, so I'm trying to figure out if it's the CPU or the M/B. SuperMicro tech support says a m/b, processor, and processor fan are all that's needed to get some activity.

My questions are, should the fans power up if the CPU is removed? How does the CPU control the PS-ON signal?

I found a replacement m/b online and have it coming, but it would be much less work to fix the existing m/b (or processor) since all my licenses will need replacement, etc. I have a bare metal backup, so I'm not too worried about restoring data.