Hts 12 not powering down.

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Finally got my Hts 12 subwoofer 5 days ago. Been having a good time playing with it getting it set up. Love it. Great addition to my system so far so good until last night.
After watching a movie I turned everything off. About an hour later I come back into the horse and discover the subwoofer did not shut itself off and it was humming. Loud! I flicked the toggle switch on the back to shut it off. It's working fine so far today.
Is this a common issue ? Anyone know the cause ?