I'm sure different amps have different designs, so specifically lets say in the context of Decware EL84 amps.

If you are playing music at lower levels (say 1/10th of a watt) vs. higher levels (not clipping, distorting, but using all of the power available) - will the tubes wear out faster, or are they 'burning' at the same rate no matter what you are doing?

I believe when you start talking about things like guitar amps where you are purposely over-driving the tubes, that will kill them faster, but that is probably not going to be happening in a audio enthusiast setting!

Thanks.

For something like the UFO amps.  Does turning up the volume/increasing output actually increase the voltage/current/heat going to the tube, or is the tube in its full-power state as soon as its powered on, and you are just using more or less of the available output capacity?

I 'think' a Class A (solid state) amp is like what I described.  Always running at 'max' and you are just using more or less of what is there.

That makes sense.

Usually when I think of increasing the output, that comes with increasing voltage, current to the output device.  Otherwise...where is the extra power coming from!?  And the answer could be 100% of the power is there all the time, and you just modulate it to anything lower you want.

Bonus question - if you start to hear clipping/distortion, does that mean you are actually hurting the tubes, or does it just sound bad?  I understand on SS amps, clipping is bad, especially as it relates to speaker drivers (and tweeters being most at-risk).