Can a head unit not play speakers correctly?

I have a pioneer premier f90bt and my front speakers will not play correctly. it woofer part of the component cuts of while the tweeter still plays. it happens only on the front speakers when connected through an amp or through the stock wiring. what else could it be other than the Hu? all wire connections are in tip top shape and connected correctly. I did have to ground my rca outter sheilds so engine noise will go away.