The dampers help deal with microphonics,& the coolers help extend tube life. Effectiveness depends on the tubes & equipment. Only you can determine their worth.
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once your system reach certain level of "refinement", then both dampers and coolers will improve some tubes better, then others...means some tubes are more microfonic or just need some damping on the outside(the amp chassis also benefits from some sort of damping...as ART Q-dampers or similar devices...)
As with all tweeks results depends on the basic quality of your equipment and your ability to discern differences.
I have used inexpensive tube dampers. They certainly did no harm, but since I use very low noise tubes to begin with I can't honnestly say they did any good either. I can certainly buy into the logic for using them.
My experience with tube coolers is similar - since the tube life of any tube is unpredictible I really have no way of judging whether or not I have gotten extra tube life. I stopped using them some time ago and have really not noticed any difference. Just my experience, YMMV.
If you get tubes that have been tested for microphonics you don't benefit by using them. There was one answer that I thought said there were sub microphonics but they are clearly not audible, whatever they do I don't know the effect on the tube or sonics.
Coolers are really probably only somewhat useful if your ventilation is insufficient after playing your system for long periods but tubes are and were designed to endure the heat they generate.
All that being said if you buy off ebay there is a good chance that they could be microphonic the simple rubber band damper will work for all but the worst tubes. Those are hopeless because the internal components create a ringing sound with vibration you just can't stop it.
As for heat it is really as important as the length of time you use the tube they wear out the longer you burn them. I am not sure what effect cooling the glass will have on the thermionic plates and filaments, most small input double triodes will outlive you anyway.
That shouldn't deter you from rolling tubes though every tube sounds a little different.
Output tubes will typically die during your use of an amp YMMV. The lifespan and screen voltage, what you set the bias at, seem to have a relationship. That makes buying used output tubes that have not been thoroughly tested for gain and mutual transductance a risky business.