Adding sparkle to the finish dramatically affects tone by embedding small metallic flakes into the finish in close proximity to the pickups. This causes EMI interference to be reflected and magnified and fed directly into the pickups. For these reasons a ground wire should be run from under these kinds of finishes to the bridge plate. Otherwise, a noise gate is necessary to stop the hum and buzzing from these kinds of finishes. However, if you have a skilled person doing your finish, they can often use reverse wound/reverse polarity metallic flakes, which turns the entire finish into a humbucker. If the refinisher wires the metallic flakes in series, it will also thicken your tone.
As a side note, metallic finishes often also affect resale value because they add a couple of ounces to the answer to the "weight?" question most often asked in the Talkbass Classifieds.
That is a VERY revealing post, thank you.