The most common factors for speaker distortion are Connections, Ground, External Amplifier and a Damaged Speaker.
Since u dont have an external amp, u can skip that part.
Make sure the connections are all ok. this also means the speakers connections via the GRIP to the pre. Next check that the ground wire of the pre is connected to a bare metal surface and screwed tight. Finally make sure ur speakers aint damaged. do a physical test. If the surrounds are ok, the other most common problem is due to dust particles that build up inside the cylindrical area around the tweeter pole fixed in the center of the dustcap. As the spider moves the cone up and down, a grinding noise is what is heard. Either blow into the speaker with ur mouth (if u want to get a face full of dust) or use a blower. Invert the speakers and tap on the magnet is another way.....