no bass from car subwoofer.. please help

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Hopefully someone can point me in the right direction, I'm in the process of building my jukebox and have decided to recycle some old car audio gear i had lying around. All was working as i want it to in the pc environment but I've run into problems with getting output from the sub.
i have tried the system exactly as it was in the car (tri-mode amp, 6in speakers, 12in Sub) and also with a 4 channel amp running the rear channels bridged. neither setup seems to output bass anywhere near what they were in the car. Can anyone who has used a similar setup please enlighten me .
the etouch PC is using realtek audio if that helps at all.
Thanks.