Have now connected car sub to pc, but still problems

I ordered the correct cable online to connect my sound card to the car amplifer, and it does its job well. However, I am still running into problems. First off, I think I may need to install a crossover or something in line from the sound card before the amplifer, as the amplifier tries to play all frequencies. There is a built in crossover on the amp at 80Hz, which I turned on, and this helped the problem, but still other frequenceies get through besides bass. Also, with the volume on the taskbar turned all the way up, the media player turned all the way up, and of course the amp turned all the way up, I am still not getting the kind of volume I was expecting out of the subwoofer. I am running the amp off of a fully charged car battery, so I know it is not a lack of power issue. I know the subs potential, and the input seems to be singlehandedly restricting it. Do I need a small amplifier in front of the input leads to the amp to increase volume? Any help would be much appreciated. I have put some time into this project, and although it seems to be growing more and more complicated, I really dont want to give up quite yet.
Thanks for your time

First of all, if the built-in crossover on the amp doesn't filter out frequencies above 80Hz (or 120Hz, depending on where you set it) then there's something goofy with the amp...you could put a simple choke on the line if you wanted, but something's not right.

Also, a typical car is running about at 14 volts usually when the car is running...your car battery is probably not pushing more than 12 volts, and a decent sized sub can actually drain a car battery pretty quickly...this is not a good setup. You should get a regular AC amp for your sub...or as a last resort run your DC sub on an AC/DC converter (inverter? I forget which). I don't think this is going to work real well as long as you're trying to run that sub on DC components though...

Also keep in mind that the room you're mostlikely using this subwoofer in is A LOT larger in volume than a car. With this in mind that car sub will never sound the same in your home with the same electronics because it takes a lot less effort to "pressurize" the smaller volume of a car, it might always sound better there. Look into buying a home theater subwoofer. The dayton 10" and 12" at partsexpress.com are great performers for their price.