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RE: Its not my requirements.

There is potentially a large difference between pure performance specs and musicality. Specs certainly do not assure musicality, and some pretty poor spec equipment can sound great. Listener impressions, like many on this thread, can be very useful (taken with caution).

IME this is everything to do about the distortion signature. If there isn't a nice 2nd or 3rd harmonic to mask the higher orders, the latter will be audible as harshness and brightness since the ear converts distortion into tonality. The 2nd and 3rd are relatively innocuous so the ear can tolerate a lot of it.

I'm of the opinion that the distortion signature is more important than the amount of distortion. The signature will say how smooth the amp is. If you can get the same signature but at a lower overall level, you can get more detail.

So SETs do fairly well in this regard because of the 2nd harmonic. But you can get a better distortion signature by going fully differential and balanced from input to output. This will cause the predominate harmonic to be the 3rd (which is treated by the ear same as the 2nd) but at about 1/10th the level. Further, because distortion is compounded less from stage to stage throughout the circuit, the overall distortion will be lower and as the order of the harmonic is increased, its amplitude will fall off faster.

The SET has what is called a Quadratic Non-Linearity and the fully differential amp has a Cubic Non-linearity.

IMO/IME the cubic non-linearity is better because its smoother and more detailed. Its also quite a bit lower distortion. For example, our amps make about 0.5%THD with no feedback while an SET does about 10% before clipping. But the SET might make 7 watts, while one of our amps might make 60, 140 or even 200 watts. If you run them at the same power levels the SET can run, the distortion is a lot lower since both amps have the property of distortion dropping to unmeasurable as you decrease the output power.

So that's one thing. Another thing to look for is that the distortion should be the same at 100Hz, 1kHz and 10kHz. A lot of solid state amps (which otherwise have great specs) actually have distortion rising as frequency is increased; their feedback is falling off with frequency due to insufficient gain bandwidth product. This is part of the reason they are bright and harsh (the other being that feedback has suppressed the lower ordered harmonics).

So yeah, the specs don't tell the whole story but we do know how the amp is going to sound based on what we can measure. Its just that the measurements you need to see never show up on the spec sheets, and for that matter what's often considered important quite often isn't; its marketing rather than engineering.

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