Tube Ring on tube preamp

just measure the diameter of your tubes go to a local O-ring supply shop and buy some silicon 0-rings the right size. couple bucks max a tube. 
they come in black or white as common colors. 

you can experiment by buying different diameter O-rings (that's like the thickness) and by putting more then one on a tube. 

anyway that will give you all the damping you should need, then its buy a less harmonic tube. 

Red Silicon ones are what I used thick as possible.
Get your size here (ebay), remember not too tight or they will over time dent the tube glass, use 1 2 or 3 per tube. And remember the smaller tubes input, phase splitter, drivers are the ones that can be more micro-phonic.

To test which are micro-phonic, raise the volume to normal (no music playing) and gently flick each tube with your finger nail, and you’ll hear the micro-phonic one/s in the the speaker quite easily.

https://www.ebay.com/sch/i.html?_from=R40&_trksid=m570.l1313&_nkw=Silicon+O+Rings&_sacat=0&LH_TitleDesc=0&_osacat=0&_odkw=Silicon+%22O%22+Rings

https://www.ebay.com/sch/i.html?_from=R40&_trksid=p2334524.m570.l1313.TR0.TRC0.A0.H0.Xred+O+Rings.TRS1&_nkw=red+O+Rings&_sacat=0&LH_TitleDesc=0&_osacat=0&_odkw=red+%22O%22+Rings

Forget about expensive BS "audio" ones.

Cheers George

I am not sure what it is you are trying to improve on the 6H30 tube but in my humble opinion, and also based on my experience using / owning tube gear for the past 25 years, there is no fixing this tube. It could be my ears but for me, and this is just me speaking but the 6H30 tube is a tube that seems to have been crammed down our throats as some Godsend when IMHO it is an awful tube. One cannot roll it, improve it or make it sound good. I am sure that I am now going to get a barrage of "You are an idiot" replies but I will take a good 6922 tube, or better yet, a good 6SN7 over any 6H30 tube. And I can honestly say I have tried the tube in some rather expensive preamps. Did not like it at all...
My two cents...and opinions, like my grandmother once told me, are like noses - everyone has one and most smell..

A good 6922 is a thing of beauty.

I wonder if I can exchange the 6H30 with 6922 on the audioresearch LS 26 ?

I use Herbies Tube dampers with excellent results:

https://herbiesaudiolab.com/collections/tube-dampers

I finally go with "Duende Criatura" rings, they are more compact, and give a better precison in sound, over standard kind of rubber ring.

You could look at the ECC99 <> 6H30 socket convertor.