If it makes good contact, you close the cage and it should always be an improvement.
If it causes hum that means something is wrong
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+1 i cant see how it would cause a problem. the entire chassis is aluminum and is grounded to sheild the "guts" of the amp. Adding the small peice on the cab is just like putting a lid on it to keep more unwanted noise out.
The only way it could cause a problem is if one of your components was touching the sheilding and sending a signal to ground that shouldnt go there.
If that happens just cover the sheilding with electrical tape except for the aluminum that makes contact with the chassis. I do this on my guitars that i sheild