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I think the purpose is largely aesthetic. But technically what they do is show the plate current, aka the bias, on the power tubes. That is useful in two ways: one is generally the needles will be perfectly still, but as you start to approach clipping they will start to dance, in time with the music.

Also, while the absolute number on the dial isn't very important, if you see it go down it probably means your tubes are getting old and you should consider replacing them. But my (some what limited) experience is sound quality starts to diminish before the meter reading changes. Steve suggests every 6 months or so to swap out your current tubes with new ones and compare the sound. If the old ones still sound good than continue to play them. If the new one are better than thank the old ones for their service and leave the new ones in the amp. That makes a bunch of sense to me.

There are really no user adjustments possible, besides switching tubes, but if you see the meters go to zero, or get maxed out that means you've got a problem.

James has answered some of my questions over the last couple years, as well as several other regulars here. Always on target-awesome folks. My output tubes are the red tip original 6P15P-EV. I recently did what James has said and swapped them out with a new pair. No difference at all so the originals are back in. We have played the amp quite a bit since new a couple of years ago. Summertime a bit less though because we are out a lot more. Pretty amazed how the tubes are holding up...  The amp is still working exactly as it should~ AMAZING . All the best! Mike  ~This lightning strike was taken from our front deck Sunday evening. I tend to unplug some things during these storms. A LOT of strikes that day and night.