Distortion on low frequency

Just because the feature is there does not mean that it has to be used. Dyohn is bang on with his advice (I was away on holidays guys  - Brooks & Dunn, Dwight Yoakam, The Mavericks and others - haha).

 What you are doing is simple. You need to think of the eq as a volume control. But first let's give you a better idea as to what is happening. You car has a redline and you know that as long as you stay below redline things are fine. Above redline, well bad things can happen. Now you need to think of your stereo having a redline that is controlled by two volume controls. If the eq is left alone you will notice that your main volume control can be turned up and there should be little to no distortion up to the redline (the point that distortion is highly noticeable). Now this master volume control is dependent on the secondary volume control, the eq (or bass / treble / boost settings). The more that these secondary controls are used in the + side adjustment, you will find that these two things will happen to the master volume control. One is that the redline will be much lower, two is that distortion is much more prevalent. This has nothing to do with how you want the system to sound, it has to do with the limitation of the equipment that you are using.

You asked about what causes it to clip. A signal must stay within a set limit, if it is asked to play outside of this limit it will clip the signal and introduce it as distortion to be amplified. When you look at todays music, I'm sure you have taken notice that some cd's tracks might have tons of bass and others do not. The first thing you do is adjust the bass settings to compensate. This can be infact part of how the material is recorded. If it recorded with a particularily heavy bass track and boosted considerably in the recording process, when the cd player is playing it back and you try to boost it some more, the signal is now hitting the redline and being clipped. In some cases it can be the cd you are playing, in others it can be that you are asking too much of the system. In all cases it will lead to the failure of a sub. Best to catch and understand it now before it is too late.

If you find the need for more bass (as indicated by the +6 setting on the eq), a larger amp, a larger sub, a different box or a combination of all 3 is in your future. I would also throw in an Eclipse cd player (couldnt resist guys - sorry dude).

Eq's can do a number of things, one is to solve a dedicated and identified problem that makes it necessary, the other thing is to screw things up, as it seems to be doing for you. Work the problem a step at a time and keep asking the questions.