My car has a Kenwood KDC-MP222 cd reciever in it. I want to get some bass in my car by hooking a computer subwoofer up to it. The reciever has stereo RCA preouts (not for sub) and I plan on hooking those up to the subwoofer. The sub has a built in crossover and a built in amp powered by 12 volts, so I won't have any trouble running it off the car battery. My concern is this:

Will the preout levels vary with the selected volume on my reciever? Or will the sub always play at the same level with this configuration? I am worried that the sub may play at constant volume even when I turn down the other speakers. I don't want to take the time to set this up if its not going to work. If anyone out there knows if the preouts adjust with the speaker outputs on a reciever please let me know. Thanks.

Yes, preout voltage varies with your headunit volume.

You're saying that the HU has 2 pairs of RCA preouts? If it doesn't have a third pair (usually for a subwoofer) then it doesn't matter - since you say your subwoofer has a built in crossover, it can take any preout signal from your HU and it'll play the frequencies that it can.

Anyway - good luck! That's a great idea for someone on a budget. Since I don't have the money yet for the 15" RE XXX subwoofer that I'm looking to put into my car, I might try that with an old computer sub from my CSW FPS2000's.