do you have your speaker level inputs connected to the wires before the amp or to the wires going to the subwoofer(after the amp)? don't know if this matters, but if it's connected to the wires going to the subwoofer, they may be putting out too much voltage when you turn the volume up. your input sensitivity is 1-16v for speaker level outs, so maybe the voltage from the amp is exceeding the 16v limit?

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