maple classic™ 
flip psu switch off and on: doesn't work
swap power cables with old psu to all but the drives: works
swap power cables back to new psu to all but drives and gpu: works
continue adding power cables from new psu to drives and gpu: doesn't work
remove power cables from all but mobo and cpu again: still doesn't work
🙃 computers.
only thing I'm worried about though is, remember how this mobo failed before? what if it was the PSU that caused that failure? what failure was it? will it happen again?
but then again it had been working with this psu since April, the psu is from 2011, it doesn't heat up or make weird noises, though i don't have an oscilloscope to tell if the power is clean or not
i did give it a good clean when i had it open back when the pc last failed though, and now the fan hardly ever even kicks in since i did that, which is a good sign?
everything plugged back in, beeps fine, drives seek, left it running for a while without incidents
so the new psu is a bust i guess?
do i trust the old one?
at the very least this time if i need to send the PSU back for a RMA i have two 500+ watt PSUs i can replace it with so no going without computer today