A couple of ideas here. If you are using a separate amp for center/surrounds, then you could use the 12V trigger output from that amp to trigger the C48 into bypass mode.
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Another idea would be to get one of those 12V wall wart DC power adapters, chop the end and solder it into a 1/8" male mono plug. Use that as the 12V trigger for C48 and then just plug it in when you want to trigger the C48 into bypass mode.
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Another idea would be to get one of those 12V wall wart DC power adapters, chop the end and solder it into a 1/8" male mono plug. Use that as the 12V trigger for C48 and then just plug it in when you want to trigger the C48 into bypass mode.