The amplifier might not like the subwoofer’s 1/2-ohm load. The amp can do okay at lower volumes, but when you turn it up the subwoofer is too low an impedance for the amp to handle and it goes into protection mode. Check to see if the amp is going into protection mode - it should have an indicator light somewhere to tell you. Even without a protection light, the amp might be shutting down it’s output because the impedance is too low. You can switch from parallel to series wiring on your sub’s voice coils. That will bring the load up to 2 ohms which might work better for your amplifier. Keep in mind that amplifiers capable of running sustained 1/2-ohm loads at high volume are pretty rare (and expensive). Your amp is probably going to be a lot happier (and more stable) with a 2-ohm load or higher.