EMI FIELDS

What you did was stop the interference by shielding the source of the interference rather than shielding the part affected. This is the preferable solution in the case where you have identified the source. From a practical standpoint, if it works as is - no it doesn't need to be grounded - you have achieved what you set out to do. If you continue to have problems, ground the conductive sheet. The question then becomes how to ground it - at one point or more. In your application grounding at more than one point should not be a problem as you aren't going to be concerned about ground loops. But, since you have solved your problem, I would not go any further. I am assuming that your interconnection between the amp and preamp is not balanced. If you the capability of using balanced interconnects then you may want to do so and thus be able to remove the conductive sheet you are now solving the problem with. If you do so, attach only one end of the shield of the interconnect to ground - at the preamp end.

Imnewton: A good source of information for shielding and grounding is "Noise Reduction Techniques in Electronic Systems" by Henry Ott. I had his first edition on my desk for a decade and bought his second edition after that. It is not a new book (copyright 1988 for the second edition) and I don't know if he has a third edition. It is practical and not math intensive- a basic high school math education with an understanding of logrithms and introductory calc is more than enough to understand the explanations, and even that is not essential.

Thank you Musicnoise. If I ground, I suppose I could run a wire from the sheet to the ground screw on any outlet? I will take your advice on the book/books!

Mario