Do not set the amplifier's gains to the max. If you're setting the amplifier gain to the max, either your amplifier is just not good enough to drive a subwoofer or you have a pretty exagerated bass that's far more than is necessary to blend with the fronts or there is some other problem (phase issue, etc).
Setting amp gain to the max is a great way to send a clipped signal to the subwoofer. In general, I prefer to calibrate the front speaker's amplifier gains such that when the head unit's volume knob is at about 75%, that's my normal listening volume. The lower you set the amplifier gains for a given music volume the better. Once I am done with the front stage, then I turn on the subwoofer and adjust its gains to blend the lows with the front stage.