The proprietor of one of the high end's more noteworthy and successful tube gear companies explained to me that the studies done in the 1950's on tube wear all confirmed that small signal tubes, such as those used then and now in preamps and now in DAC's, tend to last much longer (and as a corollary, also sound better) if powered up 24/7. Basically, he said that, if left powered up 24/7, they either die at some point in the first 250 hours (infant mortality of sorts) or they basically last forever.
I leave all of my equipment except turntable and tube amps powered up 24/7. Over the years, this has included a tubed DAC (Cal Audio) and three tube preamps (Jadis, CAT and Hovland). In fourteen years combined of these tube components being left on 24/7, I have never had to replace a tube (nine years now on the DAC), nor has any of my equipment, tube or solid-state, required service. The only precautionary measure I take is to keep the volume turned all the way down (and the mute engaged for other reasons) on preamps so as to ensure that there is little current passing through the tubes when I am not playing music.
Tube amps cannot be left powered up because the output tubes pass a relatively large amount of current that stresses the tubes.
As a general rule, it is the thermal cycles that occur when equipment is powered up and down (i.e., repeated expansion and contraction of the components as they heat up and cool down) that kills components, even tubes - it is basic physics.
