Expanding questions about tubes in a preamp

So just putting some rubber rings (O-Rings) picked up at a local hardware store would not be a good idea - or is it just again an experiment worth trying?

Better than paying for Herbies. I would try one per tube that fits snug (not real tight) and also a couple sized bigger. The big ones will let you shim or slide something between the o-ring and the tube. This will let you experiment with the vibration control effects of different materials- wood, paper, metal, whatever you want. I doubt you will find anything that does what you're looking for, but at least this keeps the learning costs down.

If you want dampers PM me I'll stick some in an envelope for you. The postage will be more than they're worth. Of course there are those who like them. There are also those who are happy to make money selling them to you. We have all kinds here.

My advice if you are curious is try some simple proof of concept tests that cost little or nothing to try. If tube damping is good you can damp a tube with a rubber band- cut some paper into strips wrap around the tube put a rubber band around it. Listen. Not something you want to leave on there very long but plenty long enough to listen and hear what it does. 

The expensive Herbies Tube Dampers I have are nothing more than a semi-circle of wire with a couple three bits of silicone that contact the tube. You could cut up some silicone made for kitchen use, its made to handle heat, try that. Point is you don't have to run around asking people, you can figure these things out for yourself and not even have to trust some stranger you never have and never will meet, who may even be so nuts he has his own website devoted to time travel and audio tweaks.