I never bought into the idea that it would affect a given guitar's tone, but, sure enough, the other day I took mine off to access all 12 poles, and was surprised at how dull it sounded.
I double-checked the tone settings and they were ok, so, I replaced the covers and got my piercing, gritty sound back in a matter of minutes.
Lets see you guys weigh in on this and other guitar myths.

Well being as though there were 12 polepieces its obviously a humbucker your talking about. A metal cover on a a humbucker or any pickup will affect the magnetic field of the pickup.

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No no and no

What surprises me is that the cover would be beneficial. I would think it would be an impediment, acting as a magnetic damper. This is not an experiment I've ever tried but have often wondered about. I'll be curious to see what some of the pros have to say.

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Just think of how awesome a guitar player you could have been by now if you had only spent the last 10 years practicing instead of obsessing over pickups and roasted maple necks.

That's the reverse of what I used to hear. According to some, removing the covers on a humbucker will improve the high frequency response. This is why you see so many guitars these days without covers on their "buckers", sometimes a mod ,sometimes the guitar comes that way. AFAIK, though this another piece of guitar lore that may or may not be true. Some people swear by it, some don't buy it at all, and some say all it does is make the pickups noisier. In my estimation, like a lot of these "myths" (for my lack of a better term) it's probably a matter of that it does change the sound, but the difference is likely negligible, it's probably more subjective than not. That's my guess, anyway. Has anyone here ever done a comparison test?