A ground fault kind of situation may well have been what fried your amp, but I wouldn't necessarily blame what happened on faulty AC wiring. Maybe yes. Maybe no.
Your new Darkglass no doubt has a three prong AC plug which is polarized and grounded. What about the sound board? If the soundboard is ancient as you mentioned, it probably does not even have a polarized two prong AC plug. Polarized AC plugs may not have been universally adopted back when that particular piece of equipment was manufactured. If the AC plug is not polarized or not properly wired for whatever reason and that old board happened to be plugged in so that the chassis was hot, then as mentioned, you might essentially have had what amounts to AC mains on the braid of the mic line between your Darkglass and the soundboard. An undesirable and potentially lethal situation.
My point is, understand this can happen even if the AC stage wiring is okay. It can happen even if the AC wiring in the booth is okay. Under certain circumstances, it could happen if one night the sound guy adds a piece of gear in the booth that happens to have faulty AC wiring. Testing AC power on stage is mandatory. Don’t let it make you think everything is okay. Maybe yes. Maybe no. How low can you go? Six feet.
If you play in bars, learn about single phase and three phase power, delta / star / Y, etc. Because eventually you’ll see every conceivable way known, and some unknown ways to screw up AC power. It’s amazing these kinds of establishments don’t burn down more often than they do. But then, that’s another story.
