Just wanted to give a big thanks and huge thumbs up to Herbie! I received an UltraSonic RX tube damper to try on my 6H30 tube for my preamp, and let me tell you... These things work!
I had tried the silicone rings in the past, and couldn't pull them things off fast enough. Just sucked all the life out of the music. With the Herbie's, this does not happen. At all! But what I do hear is tighter image focus, better clarity, and much better bass! Very surprised by this result, and I can't recommend them highly enough.
I also experimented with different positions on the tube. About 1/3rd from the top is recommended, and this strikes a nice balance. I like them just a bit lower, for even tighter image focus without losing too much ambient resolution and air.