If the distortion changes sides when you swap the cables between the preamp and amp then the problem lies before the amp... It could be in the preamp, but what kind of cables are you using? Are they shielded? If not, you could try using shielded cables. If so, maybe the shield of one cable is connected to the wrong side, in which case reversing the direction of the cable between the preamp and amp (on the distorted side) might solve the problem.
If trying shielded cables or reversing the distorted side cable does not solve the problem then the problem could be in one channel of the preamp. Also make sure that the interconnects are not lying on top of any AC cords.
If trying shielded cables or reversing the distorted side cable does not solve the problem then the problem could be in one channel of the preamp. Also make sure that the interconnects are not lying on top of any AC cords.