Tube Life: 6SN7, 6DJ8, 12AT7 et al.

Anacrusis, IME tubes degrade gradually along a fairly linear curve. So much depends on the circuit and the individual tube though that beyond that any generalizations can be wildly inaccurate. IOW, if you are looking at a particular tube product, see if you can ask the owners how well it holds up. There are forums where this can be fairly easy, for example with our stuff you can inquire here or at http://www.audioasylum.com/forums/otl/bbs.html

Independent forums are likely to be less biased.

Usually you will get more accurate data this way, and its an easier tack then the one you are taking, IMO.

Do tubes wear out faster if they are driven harder? For example will listening at higher volumes wear them out faster?

My strong guess is that volume is not a factor and that less heat and resistance might even be realized at full output.

Thanks for the really good points presented. I guess the question can't be answered in absolute terms, but I do have a better feel for how I might approach a tube purchase.

Heat shortens the life of any tube. So cooling is important, and in the case of power tubes, especially if they are in a class AB amplifier, the more power you ask of them, the hotter they get. If you don't deal with the heat effectively (some of this may be handled by the design of the amp), the life will be shortened.