There is a proper way to set the gain. If it is set properly, no problems should be seen.

An easy way to set the gain...

-Turn off all Bass Boost or LOUD on the headunit. You want to keep everything flat and level.
-Make sure the gain is as low as it can go.
-Put in a familiar CD
-Turn off your interior speakers, itll be easier if you only hear the sub.
-Find out your MAX volume on your headunit. Lets say it is 50.
-Turn the volume to 90% max, so about 45.
-Slowly turn up the gain, as you get higher and higher you will reach a point where the sub is just distorting. Stop there, and bring it back down a notch.
-Now you have your gain set. Remember not to max out the volume on the deck.