Expanding questions about tubes in a preamp

Good advice from Millercarbon.  I think you first need to decide if you like the sound you have.  If you do, just enjoy the sound and spend the money elsewhere.  Damping tubes can change the sound, but not necessarily for the better --try some inexpensive dampers and see for yourself if you must.  I have used various dampers, including Top Hats, Mapleshade (which can be overtightened and break the tubes, as I found out), Ensemble Kevlar sleeves and (the best) Andy Bouwman's octal tube dampers, and ultimately I found I liked the tubes without damping, I guess they sounded too dry and lost some bloom with the dampers, to my taste.

... 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?

Using a tube damper will not quiet a noisy tube. Noise is usually inherent in the tube. Hum could mean the tube has a fault or it’s coming from elsewhere in the circuit.

Dampers can be used to reduce microphonics from a tube. They are often used to tighten up bass or reduce excessive overtones. And they will change the sound, so experimentation is necessary.

The only time I’ve used dampers is on a component with exposed tubes. Acoustic vibration from a speaker or vibration from the audio rack can be heard through some tubes.

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.

The RAM Tube Store meticulously measures all tube parameters, including noise. They then grade each tube, and offer matched pairs and quartets. They offer a Super Low Noise 12AX7's for use in phono amps, etc. A very quiet 12AX7 substitute is the 5751, which provides slightly lower output.