Known good PSU fails upon replacing motherboard

In order to finally get USB3, I replaced my motherboard today. Everything else is unchanged (known good), including the cpu. After replacing the motherboard, the fans come on but there's no POST and no beep code. Naturally, I thought the motherboard was dead, but to my surprise when I connected everything up to an old 350W PSU instead of my normal 500W PSU, it booted perfectly! But, I can't actually use it in this form, because the 350W psu doesn't work with my gpu (which I removed for this test - I found that the 500W psu fails in the same way with or without the gpu.)

So, does anyone know what might be going wrong here? Do I need to replace the psu, even though it really doesn't seem dead? I did go through the POST troubleshooting thread already. (Incidentally, it's not a standoffs problem, since I did the spare psu test without removing anything from the case.)

Specific parts:
EVGA 100-B1-0500-KR psu
GIGABYTE GA-B75M-D3V REV 1.1 motherboard
2x4GB DDR3-1333 (1.5V)
Intel i7-3570
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