Well, I more or less ruled out pin 10 tonight - copy protected 5.25" disks are still copy protected, while non-protected disks aren't, which is supposed to not be the case if that pin is shorted. I suppose it could be intermittent.
I now suspect the VGC chip. I traced the output from pin 4 and find that it terminates at the top right pin (not sure of the #) of the VGC. I then looked up the VGC chip by part #, not initially knowing what it does, and found this thread on another site: https://68kmla.org/f...drives-bad-vgc/
Interestingly, he also thinks he might have done it himself (or rather, his son did it) hot-swapping by mistake.
I definitely feel like I'm on kind of a wild goose chase here but everything's bringing me a little closer and a little more confident in the real culprit. I no longer really suspect my PSU, I think my drives are ok, I think the Floppy Emu is probably ok. I think the two most probable culprits are either an intermittent or near-short on pin 10 of the floppy connector, or a bad VGC chip. But the fact that it happened on both boards suggests that the most likely root cause is me pulling something out when I shouldn't have.
Apparently some earlier boards, including ROM 01's, have socketed VGC's. Unfortunately neither of my boards do, so I can't really fix them. But I'm going to be *extremely* careful when I get the new board.