Describe ube sound vs solid state

Actually global negative feedback cannot to that in an amp. What it can do is force the amp to put out *half* has much power into an impedance twice as high- and therefore satisfy the voltage drive model of 'typical speakers'.

So add enough feedback to any tube amp and it can do that with ease and many do.

when you do see a spec on an amplifier that does double power as resistance gets lower is that a sign of a better design?

No- it is simply a sign that the amp has a very low output impedance and has the current available to double power. That is very different from a 'better design' which has a qualitative aspect; I prefer tubes so I think some good tube amps as often being a 'better design' than many transistor amps.

Another way to put this is our ears hear sounds without regard to how tubes or transistors work. It just happens that tubes do a lot of things that the ear finds more pleasant/less irritating, for example tube amps often have less odd ordered harmonic distortion. That is why they sound smoother, as the ear translates odd ordered harmonics into brightness and harshness.

So if an amplifier is designed to produces less of such distortion, in my book its a 'better design'; doubling power as the load impedance is cut in half has little to do with that.

Atmasphere, In that most (all?) speakers draw power with varying impedances and those impedance changes will correspond with sensitivity changes (again we're talking about typical speakers here) unless the amp(s) can change power output appropriately there will be deviations from linear frequency response.
Csarivey, the ability of an amp to double down does not necessitate the use of "...deep global NFB.". Consider Threshold and Ayre as just a couple of examples.

Thanks Ralph it was something I had believed as it is one of the most noticeable specs you see when looking at a solid state amp spec sheet.

Atmasphere, In that most (all?) speakers draw power with varying impedances and those impedance changes will correspond with sensitivity changes (again we're talking about typical speakers here) unless the amp(s) can change power output appropriately there will be deviations from linear frequency response.

Yes- and enough global negative feedback will allow nearly any amplifier to do that. Its not about being able to double power, its about the amp being able to act as a voltage source. Plenty of tube amps can do that. If we add about 20 db of feedback to our amps they will act like a voltage source too.

Again, the amp does not have to double power as impedance is halved. What it does have to be able to do is cut power in half when impedance is doubled in order to behave as a voltage source.

^It works the same in both directions, and global feedback is not neccesarily a prerequisite.