It's because you're trying to get those speakers to play frequencies that are too low for them. They aren't designed to play that low. That's why subs exist.
Without looking at that HU, I'm going to assume it doesn't have any kind of cross over function built in.
Pick up a decent amplifier with built in cross overs and set the cross over for high pass (this allows only the higher frequencies to go through). Since you're using such small speakers up front, you need to set the high pass for 250 Hz or more. If the 4 channel amplifier that you get has the ability to set 2 channels at one cross over point and the other two at another cross over point, set the rears at 120 Hz or so high pass.