I have always been rather dubious, having tried silicone (?) dampers without any obvious effect. But I read great things about Herbie's tube rings (
http://herbiesaudiolab.home.att.net/) and decided to try them. They are relatively cheap and come with a 90-day trial period. The downside is that they come from the USA - but shipping is not expensive.
I bought them for all the valves in my system (phono stage, preamp and 2A3 amp) and I am very impressed. They do the "make the blacks blacker" thing, so that the music pops out from a quieter place and has more impact. Also, noise that you weren't aware of is reduced.
I am very happy and won't go back to being without them.
There are dozens and dozens of endorsements on Herbie's web page. He is also discussed a lot in Audio Asylum and he seems like a really decent bloke.