Question about stock cd/cass HU

ok I have a 2000 mountaineer, it's just the cd/cassette with the factory subwoofer and 6disc, my question is this; does this particular HU Have some kind of integrated HPF for the front or all speakers except the sub? I running a high pass converter for signal to a aftermarket amp/sub combo I'm I'm getting a really muddy sound from them. I tried a different amp, and also a different set of speakers on both my amp and another and get the same results which leads me to believe that the signal is the problem and not the equipment. Anyone have any light they could shed for me?

The factory head unit kills bass above 1/3 volume basically. That's probably what you're getting problems with.