Have you just recently set this system up, or is this a recent issue?
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I'm not sure I understand exactly what you are saying. Are you saying that you have a Jamo powered sub, and when you turn off the power switch on the sub the amplifier that is contained within the sub overheats, until you unplug the sub from the AC and then plug it back in? If so, and if the same thing happens when the sub is disconnected from the Onkyo, the sub would apparently need to be professionally repaired.
Or are you saying something else?
Also, I couldn't find any info on a Jamo 220. Are you sure that is the right model number?