I was listening to my system and noticed that when you git close to the sub's they really sound like crap! Almost to the point where I thought it was busted. The one behind the drivers seat was doing it worse than the other. It only sounds like that when you are close to the sub, then when you move a few feet away it sound's perfect. Why?
If everything else is fine I wouldn't worry about it. If you have a subwoofer with your pc, put your ear right next to it. When I do it to mine it sounds like crap. But at normal distances it sounds fine.
The other thing I am wondering is if your hearing the stuff that's not really meant for your sub to play. Sorta like the really low tons of voices and stuff like that, but stuff that's not base. If you listen really closely, even with filters on, other stuff will still come through your sub. Filters don't usually completely cut out everything above or below a certain frequency, they just reduce the level of those frequency's. So when your really close your hearing the really quiet stuff you usually don't.