Thank you for the reply, guys.
Yesman, I'm on a Mac and the presets aren't all in the same location. The Jupiter's was the easiest to find as they were somewhere predictable - /Library/Application Support/Arturia, in case anyone else is wondering. But none of the others are easily found. Searching for ARP 2600 or Mg only gives me the applications, application folders and user manuals for the synths, presets nowhere to be found. I appreciate the info about the P and B folders, that's good to know.
Antoine, how can deleting presets make a synth unstable? I'm not a programmer, so I don't actually know any better, but that seems ludicrous.
Thanks again for the input. I'm going to keep looking and I'll report their locations if I find the rest.