There was a really cool article published in the Best Bass Gear e:bass blog, "Brass vs. Aluminum Bridge - Which Is Better?" The article notes that brass bridges (and presumably other heavy bridges like pot metal, predominantly zinc) give more sustain, and aluminum bridges are punchier (it also sounds to my ears like pot metal bridges are scoopier, and, I presume, brass also, but no reason is apparent). All I can figure out is that the higher the mass of the bridge, the more it acts as a storage device for mechanical energy. Of course, this would reduce the initial attack, as some of that energy would be stored, but later released for increased sustain. Anyway, read the article. At least it gives us SOME organized way to think about how bridges affect electric bass sound.