it could also be that your rca jacks from preamp to amp are not seated all the way in the amplifier input jacks. If you are using locking type rca connectors, then you may want to make sure they're pushed all the way in. Some input jacks make it seem like they're in all the way, but in reality there's still room.
Also, try connecting your cd player to the amp(but don't hit play) and see if that causes the same buzz as the preamp does. If not, your preamp is causing your problems.