My custom subwoofer idea... electronic wizards help me out please!!!

Ok, my logitech x-620's sub aint powerful enough for me. I'm not sure if it's the amp or the sub that is crappy... So my idea was to plug a car sub to it. Do you think the amp is crappy or the sub is rappy? because I'm not sure... it stops gaining power after a while but there is no bass distorition that much.

I know a bit about impedance and stuff... but I want to know if this is like a regular circuit and if I can measure the voltage goung to the speaker in parallel (should be 12V i think), and if I can measure the resistance becuase It doesn't say anything on my speaker about power or impedance.. If i measured the resistance (in series right?) would it be before or after the speaker that I would do this? (I mean before the positive side or after the negative).

And also, let's say mine is 8ohms, and the car sub I want to add is only 4 ohms, would adding a 4 ohm in series going to work as a compensation?

Thanks in advance for helping my retarded project!!!