Having listened extensively and also finding tubes that were dead quiet I ended up not using the rings anymore.
What I have found very interesting and would love for you to try is that I place a little of that marble i got last week on top of the tube cage. If I leave it on the top end gets a little more air and there is a tiny amount more inner detail than with it off. I now have this little 7cm slab of marble on the top all the time as I love what it does for stringed instruments as well as piano!
Also, the marble base with the 3 oak cones glued to it has proved very worthwhile for similar reasons, really makes the amp shine that tiny bit more!