You are correct. The dissipation will be approximately the same. Approximately. However, there will be a lower amount of current, mainly grid current. That does have a positive affect when lower. E.g. Whether the tubes put out 600 watts and dissipates 270 watts, or they put out 500 watts and dissipate 250 watts, or they put out 700 watts and dissipate 280 watts, there is a difference on the tubes, or solid state drives.
Tom W8JI argues that Thoriated Tungsten filaments don't care whether they are being run gently, maxed out, or even idling, they will deplete at the same rate.
What are your thoughts on that?
I'm not saying loafing is bad, or that you shouldn't run at less than full throttle... its just that there seems to be a lot of belief out there that doing so will save tubes... having also done the math, you can also see that there is no strong basis for that belief.
The best way to save tubes in an AL-811 or AL-572 is in my experience to turn the amp off when you're not using it, or going to use it in the next 30 minutes. I've depleted emission on plenty of filaments, but I have never once snapped one from thermal shock.
In fact, the death knell for the 572B that failed recently on me was when I took an unexpected nap and the amp stayed idle for 8 hours... Next time I used the amp, that tube was 50% down on emission.