Buckkoo

I have a a Z130BT in a 2007 Ford F250 with Alpine speakers and the factory sub woofer. When I turn up the volume loud the unit mutes the Alpines speakers front and rear, but the factory sub woofer continues to play. The only way to get sound back is to turn the unit off and back on ? This problem started right after installing the Alpine speakers. Their polarity is correct and they sound great at lower volume. Why does the unit mute itself ?