Maybe this isn't the same thing at all, but I had a very similar problem. I have my (OEM) head unit still and I have 2 subs with a 250 watt amp.
I also have a converter and when I got it back from Best Buy, it sounded fine. But when I turned the head unit off, the subs starting putting out a loud rumble that literally shook the entire car.
I took it back and they noticed that when the trunk (hatch) is opened, it doesn't do it. See if yours is like that too.
They looked it over and got it fixed. Unfortunetly I don't remember what he said the problem was. I vaguely remember something about looping a grounding wire
Really don't remember what he said, but it was fixed, so I didn't care.
Check if your speakers rumble when you turn your head unit off and your trunk is opened. [ 12-31-2002, 11:49 AM: Message edited by: Kyle-Cel'02 ]