Re: chrome PU covers?
The difference isn't very much, when the nickel covers are factory potted.
I think nickel is used because it has better sonic properties than chromed alloy.
If the nickel is shiny, it matches chrome hardware.
You can put covers on regular humbuckers by putting the cover on tight, soldering them on, then potting them again. You MUST use ones where the holes line up with Duncans though. Gibson covers won't work, and if you buy aftermarket, they have to specify that they can go on Duncans.