So some like more 2nd harmonics and says that the amp sounds great, when
others will even almost not want to turn it on to listening to the same
amp, when it has according to measurements not and them to high
distortion (as harmonics are).
So House sound is the amount of
distortion/harmonics. Or maybe it can be both deviating from flat line
AND have more or less harmonics?
The ear treats all forms of distortion as a tonality. The lower orders (2nd, 3rd, 4th) bring richness, bloom, lushness, that sort of thing. The higher orders bring brightness and harshness. The ear is keenly sensitive to the higher orders and really not so much to the lower orders. The ear is more sensitive to higher ordered harmonics than anything else since it uses them to sense sound pressure. For this reason a good amplifier/good system will be low in this and will not sound 'loud' even when it is. The ear is also sensitive to intermodulation distortion; the ear assigns brightness and harshness to IMD.
The 2nd harmonic brings richness, if in sufficient amount to mask the higher ordered harmonics. Amps that express the 2nd as their primary distortion product mathematically have a 'quadratic non-linearity'.
The purest form of this in amplifiers is an SET.
The 3rd harmonic is treated by the ear the same as the 2nd. Amps that express the 3rd as the primary distortion component have a 'cubic non-linearity'. Amps like this tend to have less distortion overall, since as the order of the harmonic is increased, their amplitude falls off at a quicker rate. So you don't need as much 3rd to mask the higher orders, and amps of this type tend to have 10th the amplitude of the primary distortion as opposed to amps with a quadratic non-linearity. So they sound more neutral.
Amps of this type will be fully differential from input to output.
Amps can have both non-linearities if single-ended and push-pull circuits are combined. Algebraic summing occurs, so amps like this tend to also have a prominent 5th harmonic.
Amps with a single-ended input and push-pull output usually fall into this last category, although it is possible to build an amp that has a single-ended input but is fully differential from input to output.
The use of feedback adds complexity to this picture. Traditionally, feedback suppresses distortion but adds some of its own, all of which tend to be higher ordered harmonics (plus some IMD), so it can be quite audible as brightness. If insufficient feedback is used, essentially it will reduce frequency response errors when the amp is driving a speaker since the output impedance will seem lower. But it will also be brighter due to the added distortion. Apparently the ear favors tonality brought on by distortion over actual frequency response.
If enough feedback is used (over 35-40dB) then the amp can compensate for the distortion caused by the feedback (this also allows it to correct other things, like phase shift). Because this is a bit of a trick, and because solid state amps in particular have needed feedback to operate correctly, we've seen about 60 years worth of harshness and brightness in solid state amps, which is why tubes are still around.
But recently there have been advances in semiconductors and also (more importantly) the will with designers to create **in fact for real actually** neutral amps that have almost no distortion signature (house sound) at all. IOW there are now amps that in fact have enough feedback, something new on the audiophile scene.
In a nutshell, a 'house sound', IOW a distortion signature, should be seen as a bad thing, whether its the ever-loving 2nd harmonic of SETs and many other tube amps, or the brightness and harshness of traditional solid state.
One way to minimize these signatures is to allow the amplifier to drive a load that allows it to make less distortion. In all cases this will be a loudspeaker of higher impedance. This is easily seen in the specs of all amplifiers! The speaker should also be a reasonably good efficiency, so the amp isn't being asked to make full power. SETs are a special case of this; if you want to get the most out of them you need a speaker that will never ask the amp to make more than 20-25% of full power (higher ordered harmonics show up if this rule is broken). Solid state amps don't have the graceful clipping character of tube amps (solid state amps made with Static Induction Transistors being the one exception, but SITs are long out of production) so they should always have more power than would be asked of them as well, but 10% more is often enough.
So IF sound quality is your goal, to get the most out of *any* amplifier a speaker that is higher impedance and higher efficiency should be used. If you really like the 'house sound' anyway, keep in mind its not neutral and by definition less musical, even if the amp has a euphonic character.
