And the colorations from different part selections are not the same as SET. This makes sense when you reflect on it ... the balanced circuit is cancelling most, but not all, colorations, and the residue left over can be unwelcome and surprising.
SETs produce a quadratic non-linearity, which in turn makes for a fairly prodigious 2nd harmonic. If set up properly (if you see what I did there) the succeeding harmonics will fall off on an exponential curve.
A fully balanced amp will have even ordered harmonic cancellation, so the resulting non-linearity is cubic in nature. So the 3rd harmonic will be dominant, but at an amplitude slightly less than the 3rd is when seen in an SET. Succeeding harmonics should also fall off on an exponential curve, but it will be one with a different exponent- the harmonics will decrease in amplitude faster as the order of the harmonic is increased. The reason for this is distortion is compounded less from stage to stage throughout the circuit. Since distortion obscures detail (in addition to altering the tone colors of instruments) this makes for a more detailed presentation, with less harshness and brightness than an SET can manage, which is saying something. In either case of SET or fully differential, the lower ordered harmonics will mask the higher orders.
The ear treats the 2nd and 3rd in much the same way- in that it finds them innocuous. FWIW, a properly functioning tape recorder will produce a 3rd harmonic as its primary distortion component also, so we have a pretty good indication on that alone that the 3rd isn’t a problem.
Its also worth mentioning that a fully balanced circuit, running zero feedback, will produce a greater percentage of usable power- close to or exceeding 95% of full power, while an SET is doing well to make 25% usable power.
Of course setting the correct operating point is critical in either circuit. In a balanced differential circuit, the best operating bias point will usually be just above the maximum gain that the differential circuit can do, with symmetrical clipping. If this rule of thumb is followed, there will be no unwelcome ’left over residue’. To achieve this, a proper Constant Current Source circuit should be used- a simple resistor to B- is inadequate owing to the rather low mu that most tubes have. It will be found that the current sensing resistor that is tied to B- is quite critical. I usually set it slightly high to allow for variations in the tubes themselves.
A good quality CCS cannot be made using a single tube or transistor- you’ll need at least two devices. Semiconductor CCS circuits can work exceptionally well and offer the benefit of no likelihood of tube damage as the tube warms up due to cathode filament arc-over concerns. If you work with differential circuits you find out quickly how important the CCS really is. In most solid state amps I’ve seen the CCS leaves performance on the table. If it is not optimized, the differential circuit will not achieve its best gain, distortion or Common Mode Rejection. So its important to get this bit right.