Wait, never mind. It's got nothing to do with the ignition wire.

I just checked it, and the problem is the sudden voltage loss on the overall electronics of the entire car because of the starter motor. There's a whole different thing that happens on the empeg when I turn the accessory position on and off fast. No thump that way, only when I actually engage the starter and the whole car voltage drops a bunch to power the starter do I get the thump.

Back in the old days, I had some big capacitors on the 12v constant wire from the battery to prevent this from happening. Didn't keep the empeg playing during a starter motor run, but it stopped the thump. I took it out a long time ago as part of a ground loop diagnostic and never put it back in.

So, Warp10, if you want the thump to go away, your empeg yellow wire and your amps have to be on the constant power line from the battery, and that line has to be protected by bigass capacitor(s).