The wavelength of an 80hz sound is about 14 feet...you're not going to hear it with your head right up against the sub. You'd do better to stick your head near the windshield to hear bass notes. Distance, cabin gain, and cancellation all factor into it.
http://www.soundoctor.com/freq.htm
You could have a situation where the sub in back is being canceled out by the under-seat speaker. Like James said, turn one of them off and see if your bass response improves.I can turn the level control up and it will increase the subwoofer volume but I’m not sure that is a good thing. I don’t want that muddy, overbearing subwoofer sound again.
Everyone's hearing is different, and the cutoff frequency from "sound" to "feeling" is subjective, but it's a shame to write off lower frequencies just because you can't "hear" them.I just don’t think there is that much content in typical music under 80 Hz, assuming that is where my sub is crossed over at. I’ve searched for songs that have more content in that range for testing and showcasing a subwoofer but the music I typically listen to just may not have much under 80 Hz. It could also be as much feeling as hearing the lower notes.
Having said that, if you don't have system that capable of reproducing the notes, the whole thing's a moot point.