Yea that is a pretty crapped ass system. but hey...it's free! You said only one channel works? are u sure?! well...try this..bridge ur subs. So what you're going to do is basically tie both (-) wires of the subs together and both (+) wires together. So all u did was go from 4 wires to 2 wires. Get it? That's running it parallel. Now when you plug the subs to the amp, wut u wanna do it put the (+) wire of the subs in the (+) of one side of the amp. On the other channel, plus in the (-) wire of the subs to the (-) of the amp. Try it out...see what you get. If nothing, then u just gotta use one channel.
About hearing only highs from ur subs and no bass. Take a look at the amp. What you're looking for is High pass or Low pass. Switch to the low pass. Hopefully that'll do that trick! G'luck!