Here's the thing (why do people say this? Kind of a silly phrase). With the volume peaked (~70-80% for a lot of stock systems), for the bass "hits" (let's just say they occur at 50Hz, 0dB for argument's sake), you typically need your amp to feed the sub, what... 20V to 55V? On my amp, this occurs somewhere around a 30% turn of the gain knob (I've tuned it to stay below 30V). I guess the knob turn bit isn't a totally accurate way to describe gain, since it operates on a curve instead of linearly (I think). Without using a voltmeter, how would you know? I'd bet you a cake that your amp outputs more than 2V at a 75% turn of the gain knob. I daresay you're clipping. It's worth looking into.
