i've had my bass for about a year now, and since i've had it, it had the WORST wolf on an Ab on the D string. i brought it to my luthier, and he moved the soundpost around, but it did nothing. today, i went to a local violin shop to get a replacement shank for my wheel, so i brought my bass with me. i was telling the owner about the wolf, and i played it for her. she suggested that we move the soundpost and maybe move the bridge. so we tried that. nothing happened. then someone else in the shop suggested we try the wolf-tone eliminator thing (the weight that you are talking about). we put it on, and the wolf immediately went away. we put the bridge back the way it was, and adjusted the weight, and it was still gone. when i got home, i played on it, and there is no audible change in the tone or volume of the bass.
to answer your question, yes there is a weight that can be put on the string between the tailpiece and bridge. what causes a wolf is the chamber of air inside the bass. your bass resonates at the pitch G (on the E string). when you play that note, the bass goes crazy, causing strange sounds to come out of the instrument. when you put the weight on it, you are muting that note, and absorbing the energy that makes it go crazy. so basically, there isnt a contraption that will pull more sound out of that note (at least i don't think there is...)
for anyone more experienced with wolfs and the physics of sound and stuff, feel free to correct my explanation of wolf tones. it was explained to me a year ago, there's a chance i changed things around while trying to remember.
