I use Pearl Tube Coolers. They give both a cooling and damping effect. Reasonably priced, easy to use, but they can break fairly easily. I tried Golden Dragon dampers and was very disappointed. Kept falling off the shoulders of my 300B tubes.
Art, dampers are purported to lessen some noise- a lot of tubes, particularly smaller signal tubes, are prone to microphonics- they make noise with vibration.