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.
