@bifwynne , Well, since you asked, the answer is ’distortion vs actual frequency response’.
All amps make distortion. Some distortions (IMD and higher ordered harmonic distortion) are more audible than others. Of the two mentioned, the latter is what makes solid state amps bright and harsh, and does that to tube amps as well, although they are not as harsh as solid state because there is simply less of this form of distortion.
In a zero feedback amp, there is respectively (solid state or tube) even less. That’s why they sound smoother. The problem is, solid state amps have non-linear capacitances built into the junctions of the output devices. This capacitance is responsible for higher ordered distortions (brightness and harshness) that still affects a zero feedback solid state amp.
Since the ear uses the higher ordered harmonics to sense sound pressure, it is more sensitive to these harmonics than most test equipment, since it has to cover a 120-140db range! This is an insidious, inconvenient truth that the audio industry does not like to face. This is further complicated by the Fletcher-Munson loudness curves, which place many of these upper harmonics in the most sensitive region of the ear’s response.
That makes things particularly tricky.
The reason zero feedback tube amplifiers exist is that a good number of designers have come to recognize the problem of higher ordered harmonic distortion. One fairly easy way to sort that out is to avoid using feedback, since it is known to add such distortions of its own in the process of otherwise suppressing distortion. So this means that the designer has to use other means to suppress distortion.
This results in an amplifier with a fairly high output impedance.
The thing is, if you chose the speaker carefully, the result is **far** more neutral than is possible with the conventional approach (its not subtle- you can hear it right away, whether an audiophile or not). The reason this is possible has to do with two salient facts: first, no speaker made anywhere at any price is flat. Second, the ear converts distortion into tonality, and has tipping points wherein that tonality *is favored* over actual frequency response.
The latter fact is why two amps on the bench can have identical frequency response but one will sound bright and the other will not!
I’ve found over the years and decades that if the speaker requires the amplifier to employ feedback to get flat frequency response out of the speaker, that the result has no chance whatsoever of sounding like real music! At best it will sound like a very good stereo (this bit is tricky; many people are happy with the status quo of a good stereo, because they think getting it to sound like real music isn’t possible!). Its this difference where I have drawn the line, and why over 42 years we’ve avoided feedback.
**That** is why bother with a zero feedback tube amp. Sorry for the derailment.
