i think you need to be more specific....
you mention pre out - i'm guessing you are referring to a low level non amplified output which is good however you said center channel... why would you want to connect up to a center channel output with a 4.1?
the 4.1 set only takes a stereo input? that would mean it fakes quadraphonic. it should take a 4channel or 5channel input (4 or 5 rca plugs if it uses rca).
what are you trying to do exactly? does the stereo have existing speakers and one broke while you are trying to fix it with this system or are you trying to replace all the speakers on the stereo?
you are sure they are non amplified outputs? normally the main stereo outputs are all powered for speakers being attached without any aux outputs. you need to make sure as if you connect powered speakers (the pc speakers) up to a powered output you could have problems (worst case if you have the volume levels up you could blow the speakers).
why 4.1 to the center channel? that doesnt make sense and the 4.1 has no center channel as well.
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if you expect a good answer, you need to give us more information to work on.