Re: Question regarding humbucker nickel cover
Popular consensus is that it will mellow things out in the high and low frequency ranges. The other change is the creation of an air gap between the cover and the bobbins. This is a potential source of microphonic squeal. Remedies range from the simple (packing inside the cover) to the complicated (secondary wax potting after the cover is soldered into position).
I notice that your ID is PRSguy. On older PRS Customs, the corners of the HB cavities were too rounded to accept the squared off corners of a traditional humbucker cover. The same may be true of some guitars with scratchplate-mounted, open-bobbin humbuckers.