@mahgister You got it.
what is your take on hybrid amps that use both tubes and ss at the output stage--do these amps suffer from the same issue as ss amps? or are they better at dealing with the higher-order harmonics b/c of the tubes?
Most of these designs use feedback. When you use feedback in any design, you must have enough Gain Bandwidth Product to support the loop gain (which is the gain of the amp after the feedback is added). Most amps lack the GBP to do this properly, so when they run into that limit at a certain frequency, distortion will rise on a 6dB/octave (or greater) slope. This phenomena is usually perceived as harshness and brightness. Tube amps with feedback have this problem although its not as pronounced as in solid state since tube amps tend to have better distortion characteristics (to the human ear) when running open loop.
Put more simply, hybrid amps are a partial solution. Something I've not mentioned in this thread is how the feedback is received at the input of the amplifier; in most amps that point isn't linear (typically the base of a transistor) and so the feedback signal itself is distorted. That prevents it from doing its job properly, and since this practice is widespread, has given feedback a bad rap.
Feedback isn't bad if its applied properly. In a hybrid amp, the feedback is usually applied to the cathode of an input tube which is usually a lot more linear than a transistor in a solid state amp. This does help the amp be more musical, but since the GBP is very likely lacking, does not fix the problem (you can find this out by simply measuring distortion vs frequency).
Sorry if this is a bit technical!
The reason why distortion vs frequency is so important has to do with how the ear perceives distortion. I'll give you a typical example. A common frequency where many solid state amps reach their GBP limit is only 1KHz. Imagine a signal at 1KHz; it might be low distortion, but you can have a harmonic at 7KHz that is considerably higher than that 1KHz THD figure would suggest- and the 7th harmonic is not only amusical, the ear is also keenly sensitive to it because higher orders are used by the ear to sense how loud sounds are. Sticks out like a sore thumb.
So if you want the amp to be musical, its important that the distortion vs frequency is a flat line across the audio band (or nearly so- 10KHz is probably acceptable since harmonics at 20KHz won't get a lot of attention). Tube amps with little or no feedback have no problem doing this; its part of why SETs have a foothold in modern times.
But SETs are really obsolete (IMO there is no reason for them at all); the only reason they get by is due to their distortion signature and distortion vs frequency being a flat line. You don't need the amp to be SET to do that and so there are musical amplifiers that aren't SETs (which make a lot more power and are more transparent).
But most people that are involved with SETs do not understand the facts I've laid out here. This includes a lot of SET designers. I'm sure I'll get pushback on this, but engineering is why tubes exist, why airplanes fly and why lights come on when you turn their switch on; engineering coupled with an understanding of human hearing rules is at the heart of this.