Ok on the covers, they do help depending on the type you use. They are for point to point, crosstalk, and noise reduction. Not so much the newer stuff. The older tube preamps, they can get a lot of crosstalk between different, sources. They still work quite well, just one source at a time, is an option too. Otherwise, they do dress it up a bit in the back, and keep the dust and tarnish down, on the actual contact area, until they are used.
Regards
Regards