Don't really know if they have any influence on the life of a tube, but they do help to reduce microphonics. Except for the cost, I've never experienced any drawbacks.
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I use a couple of the old brass ring/sleeve style tube "coolers" from Vintage Tube Services. The real value to me is the same that Delof gets. The microphonics are reduced considerably because they are very heavey they dampen vibrations well.
It has been argued that these older tube tweaks actually keep the heat in. In theory if you run a tube at a higher temperature than it is engineered for the filament lifespan is shortened. I roll tubes a bit so I can't say if they shorten the lifespan or not.
The tubes do not seem to get notably hotter either. The ring does dissapate some heat which may in fact, result in a cooler tube. You get vibration dampening as wellof this I have no doubt at all.
Since these are gold plated and/or highly polished they look really good- so why not. BTW Brente Jesse at audiotubes.com sells or sold a very similar product. I think VTS is no longer selling them.
I would think that if the cooler is made of a material that conducts heat better than air (which isn't hard to achieve, and brass certainly does), and it fits snugly against the glass, then it should perform like an amplifier heat-sink and cool the tubes. If however, the fit is not sufficiently snug, any air pocket between the glass and the cooler will serve as a thermal layer. I'm no scientist; it's just my two cents worth.
I want to thank everyone who has responded thus far;I think I will give them a shot;I just traded in my cary slam 100's for audio valve challenger 180's and was asking for people who had experience with these monoblocks;one of the responses recommended pearl tube coolers;so that brought forth the question on coolers.