Power output of tube amps compared to solid states

Unsound, I think the 'harm' comes in when the speaker requires that the amp use a lot of feedback to work right with the speaker. Since our ears use the 5th, 7th and 9th harmonics as a means to perceive sound pressure, and since negative feedback is known to audibly enhance (distort) these harmonics, the result is something that sounds more like electronics and less like music.

This harmonic distortion contributes to the amp sounding 'loud' or 'shouty', even though it might be nowhere near clipping. Now we get back on-topic: tubes, which generally use less feedback, tend to have more *usable* power as they tend to sound less 'shouty', and thus encourage you to turn the volume control up higher without stress. This assumes, of course, that the tube amp is able to drive the same speaker that the transistor amp can, for any kind of reasonable comparison.

IME if the speaker is has reasonable efficiency, and lacks weird phase angles and the like (IOW is not demanding of the amplifier), that **all** amps driving that speaker will sound better. BTW this is a strong argument for higher impedances, since even though a transistor amp might be able to drive a 3 or 4 ohm load without strain, that is not the same thing as saying it is sounding its best while doing so.

It seems there is no single model or paradigm that can account for all possible combos of amp and speakers that work well together. Usually, two things work well together because they were designed to do so. So it all boils down more to the desires of the designer and how well that matches to those of the listener. There are certain engineering principles that can guide this process to a happy conclusion, however in the end no two designers or listeners are likely to share the exact same goals, which is why the search for a single absolute truth in audio is a fruitless one.

A bit off topic here, but, if a speaker manufacturer needs to veer from the criterion you use to describe a "decent speaker design" and needs use something other than a higher impedance amp to achieve better results, what's the harm?

Ah, Unsound . . . this is the crux of the matter. Atmasphere and I have actually beaten this horse pretty dead into the ground on other threads . . . again, I'll try to summarize:

On one hand, I feel that one of the most important hallmarks of good engineering is to carefully consider the application, and design the equipment to work at its best within it. So a loudspeaker designer should consider the types of amplifiers that are likely to be driving the speaker, and design so as to acheive the most consistent and best results over the greatest possible number of situations. An amplifier designer should do likewise in consideration of the types of loudspeakers that are likely to be connected to it. If this approach is followed, then if a person buys a "great-sounding amp", and a "great-sounding pair of speakers" . . . then there's the highest likelihood of getting a "great-sounding system".

I also personally feel that if an engineer's preferences for certain circuit topologies get in the way of these goals . . . then the engineer should maybe reconsider their preferences. Expecting a consumer to anticipate the effects of non-standardized technical equipment interface criteria is unreasonable, or at least very unlikely.

On the other hand, Atmasphere feels that benefits of his particular circuit design approach are so great as to tolerate the fact that there will be some inconsistencies in performance depending on the particular application. To me, this is like making wines that can be wonderful and complex, but are inconsistent between vintages . . . the requirement is then on the purchaser to have some sommelier skills to compensate for it.

And it's not that I necessarily have a great many circuit preferences that are contrary to Atmasphere's . . . I love the elegance of good vacuum-tube circuits, I think that there are good applications for well as open-loop topologies, etc. etc. It's just that I would never build and sell an amplifier with such a high output impedance, knowing that inevitabily a certain percentage of the time somebody would connect it to a loudspeaker that's largely incompatible . . . and I wouldn't be happy with the way it would perform under those conditions.

Atmasphere, I'm not sure that many speakers actually necessitate that the partnering amplifier use feedback.
You say that the use of negative feedback contributes to the amp sounding "shouty". I find that ironic, in that "shouty" attribute is one of the most prominent ones that I find so objectionable in the speakers that are usually paired with tubes.
It would appear to me that in some case lower impedance would offer benefits that the speaker designer feels outweigh whatever negatives that tag along.
Again, I suppose this where we disagree. I would suggest that if a speaker manufacturer builds a superior speaker that requires an amplifier with a particular set of parameters to be effective, then so be it, make and use that amplifier. You on the other hand seem to suggest; that I can make a superior amplifier so long as the speaker works within it's parameters. As I feel the speaker/room interface presents the most challenges for the prospective system builder, I would propose; that the speaker be the determining factor in determining an amplifier/speaker interface. You, on the other hand would propose; that the amplifier(s) should be the determining factor in an amplifier/speaker interface. I suppose we have a conundrum, as to just who is seeing the forest from the trees?:-)

Two different recipes that both can produce some very good soup.

Atmasphere's is a more unique recipe I'd say that might deliver some very special results if followed properly.