In a circuit, the transistors can be under several sets of conditions, which influence the amplitude and nature of the distortion being generated. If you were to look at the transfer function of a circuit, plotting input on the X axis and output on the Y axis, the more it looks like a straight line from input to output, the less total distortion. But the sharpness of any bends or kinks in the transfer function (its second derivative, to be more accurate) determines the order of this distortion.
A smooth transfer function with low second derivative, looking somewhat like a portion of exponential or a parabola (left) will generate low-order harmonics whose amplitude decreases as signal amplitude decreases. This is typical of small signal class-A stages.
However a transfer function with sharp bends or kink (right) will have higher second order derivative, and will generate more high-order harmonics that are more difficult to get rid of with feedback due to falling open loop gain at high frequency. If the kink is near zero amplitude, then the distortion amplitude won't necessarily decrease with decreasing signal amplitude. This is typical of class-AB or class-B stages.
So, your transistors can be used in:
Small signal: variations of both Vce and Ic are small compared to the bias point. This is the most linear condition, like the curve on the left.
Large signal: large variations of Ic, to the point of turning off the transistor. This will generate the most distortion, and it produces high-order harmonics due to the sharp change in characteristics near turn-off. This is like the curve on the right.
But there are also:
- Large variations of Vce, which means Early and other effects can't be ignored, but small variations of Ic, so the exponential characteristic remains quite linear. This generates a higher amount of low-order distortion.
Here's a typical low-end opamp, JRC4558. Link to image source and circuit analysis.
In this circuit, unless the opamp is clipped or slewing, the high open loop gain means the input signal of the input stage is tiny. So Q2-3-4-5 work under small signal conditions, at least for current.
Q2-3 Vce can vary quite a lot if the opamp is in non-inverting configuration, which introduces a bit of low-order distortion. Some opamps fix this by adding a cascode to the input stage.
Since S2-3 have no emitter resistor, the transconductance of the input stage will be a tanh function, only linear near zero input signal. This type of input stage works best on small signals, which means it requires large open loop gain. If the signal has too sharp edges, using a significant proportion of available slew rate, or is at a high frequency relative to the available bandwidth, then the input stage will have to output more current to charge compensation capacitor C2. This requires a larger differential input signal, which causes an excursion out of the linear center part of the tanh characteristic, and thus more distortion.
Q6 works as a current gain stage, again small signal.
Q10 also works as current gain stage, with large voltage swings, so it will show the effect of varying hFe with Vce and self-heating. This
The output transistors Q11-Q12 work under large signal conditions because in normal use, they are expected to turn off as the output stage goes in class-AB. In addition, they see large current swings, so it is important the transistor has hFe constant enough with varying current.
The output stage is usually the largest source of distortion in the whole amp, simply because it is the only part that works under large signal conditions. All the other transistors work in class A. The only way to mitigate that is to remove the emitter resistor to get a better transfer function, but this is only possible if thermal runaway is avoided with a tight thermal tracking circuit. Otherwise, the only thing available to correct it is feedback.