Hey cathead.
The answer to your question is very physics related and is the opposite of what many people may think. When you have something wedged between the body and the tail piece, you actually stop the tail piece from vibrating at all. One would think that if you stop extra things from resonating, then the bass would be softer. However, if you prevent all vibrations, except for the string you are playing on and the body, then you'll get the loudest sound. The reason for this is energy input and output. When you stroke the bow accross the string, the string vibrates. Those vibrations go into the body via the bridge and create sound. But, everything on the bass resonates, such as the tail piece, the scroll, etc. So, you INPUT energy with your arm and the bow to the string, which OUTPUTS energy with vibrations, thus creating sound, but, the outward energy is distributed all through the bass. When you dampen the tail piece, you suddenly get more vibrations in the body and in other places of the bass. Here's the key: every part of the bass creates sound BUT, the hollow body and the sound post create much more sound than everything else. Put your finger on a note and bow ABOVE it, close to the nut: you hear a sound, but that resonance lies in the neck and scroll and the string, with a tiny bit in the body of the bass. Thus, you get a tone, but it's silly and soft and thin (fun too).
Another experiment to try: Play a G on the D string (same pitch as the open G). WHen you play it, the open G resonates a TON. INPUT and OUTPUT stuff again. You input energy, and you create a note, but the G string resonates a lot because the vibrations match up. Because of the matching pitches, you have an additional output of energy into the open G string.
NOW, keep playing the note and collapse your left hand on the open G string dampening the string from vibrating on its own. What do you hear? The note is a lot louder and probably has much more depth. This is because the open G is using no energy for its vibrating, and he body gets that much more energy, which produces that much more sound. I know that the resonating G creates sound too, but the body produces much much more.
Harmonics in all of the strings resonate, tail pieces resonate, necks and scrolls resonate. These all contribute to the sound of a bass. I'm not saying that you should dampen everything that you can to get a huge sound. When you do that, you loose SUSTAIN. If you to the trick with the G on the D again, but with pizz, then you'll notice the same increase in volume and depth, but you'll notice a DECREASE in sustain. If you dampen everything and play with the bow, you'll notice that the bass fights a little more, but the sound is bigger and deeper. Again, you input energy and it is all outputted into the body, and to make a big bass body resonate takes a lot of work.
So there's a trade off: Ease of playing and sustain vs. that massive bass sound. The bass on its own is a good balance. But people always tweak things to their own prefs. The all wire tail piece, the ball wedged in, even that little weight clam that you put on he head stock of a guitar to boost sustain; all of those things deal with the same concept.
Sorry about the long post, but it's good info to know and people often think the opposite. So play around with stuff like this, tell us what you think, share results, etc.
Peace,
Charlie