Look at your amp, you probably have it on HPF(High Pass Filter) or some kind of filter that allows everything to come through it. Switch this to LPF(Low Pass Filter).
It also depends on the artist. Some of them may be deep enough to be heard.
If you hear Tiny Tim through your sub, you have major problems. //content.invisioncic.com/y282845/emoticons/biggrin.gif.d71a5d36fcbab170f2364c9f2e3946cb.gif