I have had this problem for ages. I think a good number of other people do, too. I had thought that it was due to a bad leaf switch in the mains barrel connector. I soldered a jumper across the appropriate pins so that it thought it was always in DC mode. This did not solve the problem.
I got in my car this morning to a dead battery, so I figured it was time to attack the issue again, and I think I might have found the actual culprit this time.
When my ignition is off, my accessory line floats. This could cause a transient voltage between ACC and ground — enough to make the empeg think that it needs to turn on.
I'm going to try connecting a resistor between ACC and ground and see if that solves the problem. Can anyone see a problem with trying that?