I've heard of using a small time delay relay that keeps the HU on for and extra 1/2 sec. longer than the amp when you cycle the key off...but that could get a little involved and don't want to jump to conclusions. You might only need a small bleed off resister to allow the amp to power down.
Is there any powered up light on the amp?...I'd be interested to see if you can notice if the amp shuts off before/after the head unit and get an idea of just when you get that popping occurs. Any research I've done on this issue never showed any clear cut "cure" for it...usually involves a little trial and error. Surprised ARC had no idea's for you??