Dumb question -
Why do amplifiers get hot or warm? Are they specifically designed that way? Is there some sort of audio benefit to dumping excess energy as heat?
Tubes also have heaters, they'll run hot with no bias at all. Class A solid state amps have the same issue of running hot all the time because of the devices being biased "on". There are plenty of modern amps that run cool, but people are afraid of new tech and think it's all bad, so they often don't get into high-end products. There are also old designs that run cool-ish. But part of the high-end is the heat. You can feel it, it's doing something, and don't they all sound "warm"?
I do like the look of these, though:
A pair of 833A tubes biased Class AB will do over 1kW. Those are 833C tubes, probably Class A, 200Wpc. The red plate glow is normal, and yes the do heat your room.
It's funny. Coda used to make a nice little 25 watt class A amp. And they found there were a surprising number of people who would say "I want class A! I want class A!" Then when they got their class A amp, they'd say "Ooooh, it's hot! It's hot! Take it back! Take it back!"
They still have the power consumption specs up. The new Asgard's rated at 30 watts, the original at 35 watts.
Keep in mind that the original Asgard was a single-ended amp (Asgard 2 may be as well but it's not stated specifically as was the case with the original), so around 25% efficiency. And they're using MOSFETs so the rails are probably a bit higher than would otherwise be the case. And for a given quiescent current, the higher the rail voltage, the more power you're going to consume.
Yeah there are sometimes audio benefits of running certain components with higher voltage rails, operating them more in a more linear region, etc. which increases power consumption. More power supply filtering can sometimes help, and that takes power as well.
However, there are alternatives and other designs people can use that are much more efficient, and some of these sound just fine, or very good. Why doesn't everybody just do this? It might be more marketing than anything else, meeting expectations and wants of the market, wanting to do something different or more traditionalist / retro.
To my way of thinking, designing the best A/B amp would be one that requires an extremely small amount of class A to behave and a power supply of correct size for the task. It should idle at near room temp unless the transformer is crap and only get warm in use.