Leave it at high, or blow your sub. You've been told this by everyone here, multiple times. What more do you want? LEAVE IT AT HIGH. LEAVE it at HIGH. Leave the input voltage, set to high. Leave it at high. Keep the input setting at high. Leave it at high. Your amplifier input setting needs to be on high, to properly match your headunit's output voltage. Leave it at high.
Yes it will be quieter, and that's ok. That means you aren't over-powering or burning up your equipment. Leave it at high input. Set the gains properly, with it set on high-input. If you want to do anything else, do it, but please don't ask again. You have a $600 woofer, and you've been instructed how to tune it properly so that it is getting the optimal amount of power, and a clean signal, for maximum lifetime. Leave it set to high. If it is not loud enough, there are many other woofers, for cheaper, that will fulfill and surpass your needs.