The gain control is meant to be set to match your HU pre out voltages. If it's set above this, you have chance of sending a clipped signal. This clipped signal will fry your sub and can potentially damage your amp.
Optimally, you'd have the amp set to 2.2v. I believe there is information on setting it at www.bcae1.com. Just be careful once set, moreso when playing at higher levels. Listen for distortion, smell for a burning smell incase you're clipping.