You have side-stepped your bold declaration that tube amps, the entire category bar none, are on their way out.
@fsonicsmith That was not my intent. To my understanding I've not side-stepped this issue at all:
IMO power tubes are on life support- even in the guitar industry class D has been making significant inroads in the last couple of years. Its clear that any manufacturer of amplifiers now has to contend with class D technology in a way that they did not a decade ago; in particular manufacturers of tube amplifiers will find their market shrinking dramatically over the next ten years. The reasons for owning tube amps (the 'sound') is being heavily eroded by advances in class D technology; any manufacturer that ignores this does so at their own peril.
Would you please expound upon your statement: "keeping distortion vs frequency linear across the audio band"? I am afraid I don't quite follow.
@atulmajithia In order for an amplifier to exhibit the same smooth character as the music itself, in order to not sound bright, distortion must not rise with frequency. This is one of several characteristics needed to allow the amp to be musical and not bright. If a tube amplifier is zero feedback and has sufficient bandwidth (not including the output transformer if one is present) then this really isn't a problem. Of course the designer needs to pay attention to other issues but distortion vs frequency is pretty important.
The ear is very sensitive to higher ordered harmonics since it uses them to sense sound pressure. It also assigns a tonality to all forms of distortion and higher ordered harmonics get the value of 'harsh and bright'. In addition distortion products occurring in the Fletcher Munson region (3-7KHz) causes distortion in that range to be more easily heard.
TIM is a product of an amplifier having high feedback but also has part of the amplifier outside of the feedback circuit so unable to compensate for certain types of distortion. An example might be the base of a transistor in a differential pair, wherein the input signal is applied to that transistor while the feedback is applied to the base of the transistor's mate. This is a common circuit in many solid state amps over the years. Class D amps of the self-oscillating variety (like ours) don't have that kind of input circuit and so avoid this problem.