Assuming you've tried turning the subs gain control down and the amsp sub channel control up to provided a stronger signal, I'm afraid it sounds like a faulty one.
If returning it is a problem, you'll have to get used to turning it on and off manually when required or use remote control power sockets like
these.
In the end, problems like these seldom get better, so I'd return it ASAP which will leave you with more of your warranty left to cover this, or any other problems that may arise.
Russell
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