Does it happen with all interconnects and all inputs? I would have to say if yes it is with the input jacks. Did you get the pre-amp from a store? If so I would bring your ic's back there and give them a try.
Also I have seen some ic's where the cable is a much larger diameter than the plug and if that is the case and the jacks are close together it may be that when the cables are pushed all the way in the plug on the cable could be going into the jack at an angle because the thickness of the cable jacket.
Also I have seen some ic's where the cable is a much larger diameter than the plug and if that is the case and the jacks are close together it may be that when the cables are pushed all the way in the plug on the cable could be going into the jack at an angle because the thickness of the cable jacket.