Base, roof and in-between
Metal parts used in humbuckers, like baseplates and covers can be really disastrous for the sound, since these may significantly increase eddy currents and thus decrease the frequency response curve of the pickup, especially in the midrange and treble areas. The effect appears mostly (but not only) on cheap pickups. Further metal parts are made of HQ steel alloys.
Metal pickup parts such as covers (which are found in Telly neck and humbucker pickups), base plates, pole screws, and slugs (mostly found in humbuckers and P-90 pickups) will affect the overall guitar tone as shaped by the pickup. Due to the parasitic eddy currents found on the surface of such parts, the midrange and higher frequency ranges will be mostly affected.
For that reason, at GAS Pickups we use high-quality metal parts mostly made of "bad" electric conductor material (as German silver), which will cause the least possible current leakage and therefore the least high-frequency damping.
The metal cover of a humbucker is one of the most important sound-affecting parts on a pickup. Cheap covers, usually made of brass (chrome or gold covered), show a tremendous cut of the high frequencies due to the high content of copper (which is a very good electric conductor). German silver-made covers are of much better quality since they are made of a material with a lot higher specific electric resistance, which does not allow but weak eddy currents to show on its surface. However, German silver covers that are chrome or gold plated show a different high-frequency response (with more damped resonance curved and lower resonance frequency value) than raw, un-plated ones.
This is a fact that should be considered, especially when ordering a humbucker. Most of the users decide whether to buy a cover or not, following strictly cosmetic criteria (how it looks better). Covered or uncovered, however, may have a significant impact on the upper midrange and highs, as explained above.