I also previously found that moving the rings to the uppermost top of each 6H30 to be the best position, and have also removed the rings from the power supply tubes (I have Ref 40 which utilizes 10 6H30 tubes, two of which are in the power supply, one for each channel). I tried herbies tube dampers as well, but the rings were better sounding in my system. I also found that two rings as supplied by ARC were better sounding than a single ring on the tubes in my system.
Tube dampeners on ARC gear ... FREE TWEAK
Here's a cheap (free) tweak for owners of ARC gear using the black tube dampeners. If you move both dampeners as far up toward the top of the tube as possible without having the top one fall off, taking care that both dampeners are solidly touching each other, you will get better dynamics, bass, mids, highs and a lower noise floor. Try it ... you'll like it.