Best tube dampers that you have used

I've used Herbies for small and large power tubes and, frankly, I honestly can't tell the difference. I've gone back and forth a few times, no difference. I leave em off now so as not to compromise cooling in any way.

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.