Typically a turn off pop is caused either by a grounding issue, or just for the simple fact that the amp needs to turn off before all the other equipment.
To confirm it is not a grounding issue, the first thing to do is try the different jumper positions on the unit. There are three different positions. The unit will ship in the “isolated” position. You can find the location of the jumper of your particular unit here.
AudioControl always recommends you connect your ground to the same point as all other equipment in your install. Ideally, you should ground the unit back to the negative terminal of your battery.
If grounding checks out, the other option is to ensure the aftermarket amp turns off first. The easiest way to achieve this is installing a capacitor on the “remote in” wire to the unit. This would allow the unit to stay on a few seconds more, which should give enough time to the amp to turn off before the other equipment. It is also good to note, for this fix, you would not be able to use the GTO signal sense feature.