Re: Tube life when running low?
This depends on many factors. Power amp stage: Tubes life will be affected by how hard they're working. Home playing volumes will probably last longer than the same amount of working time playing live at 8-9 level. But. And ther's always a but. If its a class A style amp, then even just having the amp on, it's working them tubes 100% anyhow. regardless of how much the speaker is moving.
Heat, type of amp, ventilation, etc will all have a hand in things, even number of on/off switchings will be involved.
In the larger scheme of things, pre-amp tubes should last much longer. (quite a few years) Power tubes less so (2-3 years in the bedroom, or much longer even)
I'd say that preserving tubes is not worth it. Get a nice set in there, a back up set on the shelf. And rock as you wish. Don't baby the valves. Work 'em. Go to eurotubes.com , get the JJ's, they're not expensive and are consumables... like the tyres on a car... their cost doesn't prevent driving to the coast.
matzos