You probably can't. I'm not 100% sure, but I think the crossovers for the speaker may be in the amp/sub/control module. If you hook them up directly, you will have problems... if I'm right.
You should be able to find a cheap but decent sounding set of bookshelves for $50-60.
You should be able to find a cheap but decent sounding set of bookshelves for $50-60.