In my search for a set of component speakers I see they have a setting for +3db 0db & -3db. Some I have seen are just 0db or -3db. What exactly does this do? I'm assuming it changes the sound and probably increases (+3b) or drops (-3db) the sound level of the tweeter but we all know happens when we assume.
You assume correctly, its a level control thing. Tweeters are very efficient, so in most cases 0db or -3db are the typical settings. Its extremely rare that we need a +3db.