Best tube dampers that you have used

For small octal tubes Andy Bouwman of Vintage Tube Services made some very heavy brass dampers that were extremely effective.  Not sure that he still makes them, you should check with him.  I might be selling mine at some point, I have more than I need. 

I use the Herbies Rx in my ARC Ref40 and the HAL-O in my VTL MB450III. They are well made and very effective but they will change the tone and presentation of your amps. If you are looking for warm and tubey sound don’t go with the Herbies. They definitely bring out the crispness and definition in my gear but this may not be what you are looking for depending on how the rest of your system is configured

I used some sorbothane-rings as tube dampeners about 20 years ago. 

Almost all tube dampers actually hurt the sound. But we’re so convinced Tube dampers are a good thing we leave them on and don’t think twice about it. Big mistake. Most dampers overdamp or have some odd side effect. Herbies are the only ones that improve the sound imho. With some caveats. The trick is where on the tube to place a Herbies damper. You can’t necessarily go with what people say. The best place is usually - but maybe not always - right where the getter is on the tube. So if it’s a top getter the damper goes near the top of the glass. It’s it’s a bottom getter near the bottom of the glass. For large tubes with bases another Herbies can be placed on the base for further improvement. Obviously, it is possible in many cases to spend more on Herbies Dampers that you did on the tubes. 😀