Sugarbrie,
I certainly hope that when you say everything should be plugged into the preamp, that youre not including the power cord as well. You should never connect the power cord from your amp into the power outlet on the back of your preamp. However, there should only be one power cord in the entire system that should be grounded, weather it is the amp or the preamp doesn't really matter. I mentioned the amp because generally it is the amp that has a ground plug.
I certainly hope that when you say everything should be plugged into the preamp, that youre not including the power cord as well. You should never connect the power cord from your amp into the power outlet on the back of your preamp. However, there should only be one power cord in the entire system that should be grounded, weather it is the amp or the preamp doesn't really matter. I mentioned the amp because generally it is the amp that has a ground plug.