Your thread title asks for some physics insight. I was trained as a physicist and have a degree in physics. The people telling you that the plastic cover is wholly and completely ignored by the magnetic field are correct. As far as the electromagnetic action of the pickups are concerned, everything non-metallic on the bass does not exist. Any metallic cover will have an influence, thanks to Lenz's Law. A ferric metal will have a bigger effect of course, as then there are direct magnetic alterations to the field as well as Lenz's Law effects. A non-ferric pickup cover like aluminum or copper or what-have-you will only have the Lenz's Law effects. line6man mentioned this effect on page 1, and he was correct about it, as usual. When the string vibrates and alters the magnetic field it passes through, that changing magnetic field field will induce a current in any metallic pickup cover (whose design allows for a closed loop). This induced current will create a magnetic field in opposition to the original field, not cancelling it, but definitely reducing it. One of the great shielding guides on this forum showed a method for shielding the insides of pickup covers with copper tape. The creator of the guide was hip to this effect because he didn't connect the tap all the way around the inside of the cover: he left a gap on one side so that the tape wouldn't form a full loop. This will greatly reduce back-EMF (Lenz's Law) effects, though I doubt it'd remove them completely. Bottom line: If you want to surround your pickup in metal between it and the string, prepare for tonal differences from an "invisible" plastic or wooden cover. Cheers!