I wish Bob Latino would weigh in on this, but I, too have been labouring under some popular misconceptions about tubes and tube life and have been set straight by some of the wiser folks on this forum and elsewhere, so I'll pass along what I have gleaned from speaker-makers and tube resellers:
As a general rule-of-thumb, a tube is like an incandescent light-bulb. The longer it burns, the more prone it is to failure. (Caveat: tubes do deteriorate over time, unlike a light bulb, which either works or doesn't.)
Tubes run full-tilt all the time. So volume settings are irrelevant to their longevity. The significant variable affecting tube life -- as Kent mentioned -- is bias. If 0.6v/tube works, try .55 or even .5. How many more hours you get by lowering bias is not something I know, but if you want full-time
Class A at a higher-than-recommended bias it will cost you tube life. How much, I cannot vouch. I had a modified M-125 run for perhaps a month at twice the "normal bias" on the KT-88s (Genalexes) and there were no failures other than whatever died to double the bias. Per Bob's advice in the owner's manual I check tube bias monthly, so the double-bias thing could have erupted anywhere within those intervening 30 days. I doubt they ran at double-juice for a whole month, but there was no visible or audible impact on the machinery or the sound -- in other words, they didn't red-plate or sound weird. (Being on the cautious side, I chucked the quad; a $400 cock-up.)
Again, what I have been told is to expect up to 10,000 hours out of a pre-amp or driver tube, and 2-3k hours from output tubes. I have heard people say you can squeeze 10k hours out of an antique Tung-Sol or GEC KT-88/6550-type output tube, but I bet not, given the higher plate voltages that our Dynaco derivatives operate on.
A personal tube myth of mine has been debunked, as applies to powered subwoofers. Powered subs do NOT alleviate the load lifted by the power tubes. I had ASSuMEd such would be the case but it is not, whether the cross is made between the pre-amp and power amp Vandersteen-style, or directly at the speakers, Tyler-style. Your output tubes are running full-fart, regardless.
So the only variables I can discern are bias settings, and actual operating hours. As the planet's tube pro, Andy Bowman, is fond of saying, "Don't turn your back on the bastards." If you're not actually listening to them, turn them off. That applies to pre-amps as well as to power amps. A few old-timers say they leave the pre-amps on all the time, as they do take an hour or two to warm up. But as Roy M. would say of pre-amp tubes, "How many electrons do you want to waste? There are only so many of them in a tube."
Let's face it: the price we pay for good sound is in the cost of the consumables. Get over it.